diff --git a/README.md b/README.md
index 375f11d..4cff208 100644
--- a/README.md
+++ b/README.md
@@ -62,7 +62,11 @@ like an explanation, expand the small **Why this result?** detail.
 
 Light and dark themes follow your system preference. Keyboard navigation,
 short transitions and reduced-motion support keep the interface familiar
-and comfortable.
+and comfortable. In **Settings → Appearance**, choose a solid, glass or
+frosted-glass window background and adjust its tint. Reading surfaces stay
+clear. Native glass currently requires **Windows 11 22H2 or later**. macOS,
+Linux, unsupported Windows versions and high-contrast mode currently use a
+solid background; your appearance preference is retained.
 
 ![searchi finding and previewing a mountain lake photograph](docs/screenshots/media-search.png)
 
diff --git a/apps/desktop/src-tauri/Cargo.lock b/apps/desktop/src-tauri/Cargo.lock
index e422654..7ea0f22 100644
--- a/apps/desktop/src-tauri/Cargo.lock
+++ b/apps/desktop/src-tauri/Cargo.lock
@@ -3823,6 +3823,8 @@ dependencies = [
  "tauri-plugin-opener",
  "tempfile",
  "uuid",
+ "window-vibrancy",
+ "windows-sys 0.61.2",
 ]
 
 [[package]]
diff --git a/apps/desktop/src-tauri/Cargo.toml b/apps/desktop/src-tauri/Cargo.toml
index 3496321..f14edb0 100644
--- a/apps/desktop/src-tauri/Cargo.toml
+++ b/apps/desktop/src-tauri/Cargo.toml
@@ -30,3 +30,7 @@ searchi-media = { path = "../../../crates/searchi-media" }
 searchi-ai = { path = "../../../crates/searchi-ai" }
 uuid = { version = "1", features = ["v4"] }
 image = { version = "0.25", default-features = false, features = ["jpeg", "png", "webp", "bmp"] }
+
+[target.'cfg(windows)'.dependencies]
+window-vibrancy = "=0.8.1"
+windows-sys = { version = "0.61", features = ["Win32_UI_Accessibility", "Win32_System_SystemInformation", "Wdk_System_SystemServices", "Win32_Graphics_Dwm", "Win32_UI_WindowsAndMessaging"] }
diff --git a/apps/desktop/src-tauri/src/appearance.rs b/apps/desktop/src-tauri/src/appearance.rs
new file mode 100644
index 0000000..4a3babe
--- /dev/null
+++ b/apps/desktop/src-tauri/src/appearance.rs
@@ -0,0 +1,128 @@
+//! Native materials are applied on the UI thread. Unsupported systems stay
+//! opaque; stored preferences are retained for the user's next supported session.
+use crate::settings::BackgroundMaterial;
+use serde::Serialize;
+use tauri::WebviewWindow;
+
+#[derive(Clone, Debug, Serialize)]
+pub struct AppearanceStatus {
+    pub requested_material: BackgroundMaterial,
+    pub effective_material: BackgroundMaterial,
+    pub opacity: u8,
+    pub native_effect: bool,
+    pub fallback_reason: Option<String>,
+}
+
+pub fn apply(
+    window: &WebviewWindow,
+    material: BackgroundMaterial,
+    opacity: u8,
+    high_contrast: bool,
+) -> AppearanceStatus {
+    let mut status = AppearanceStatus {
+        requested_material: material,
+        effective_material: BackgroundMaterial::Solid,
+        opacity,
+        native_effect: false,
+        fallback_reason: None,
+    };
+    if high_contrast || material == BackgroundMaterial::Solid || opacity == 100 {
+        #[cfg(windows)]
+        {
+            let _ = window_vibrancy::clear_acrylic(window);
+        }
+        if high_contrast {
+            status.fallback_reason = Some("High contrast uses an opaque background.".into());
+        }
+        return status;
+    }
+    #[cfg(windows)]
+    {
+        if !native_backdrop_available() {
+            status.fallback_reason = Some("Glass requires Windows 11 22H2 or newer with desktop composition enabled and high contrast off. Using a solid background.".into());
+            return status;
+        }
+        let _ = window_vibrancy::clear_acrylic(window);
+        // Keep the webview clear; the frontend draws a theme-aware readable tint.
+        let result = window
+            .set_background_color(Some(tauri::window::Color(0, 0, 0, 0)))
+            .map_err(|error| error.to_string())
+            .and_then(|()| {
+                match material {
+                    BackgroundMaterial::Glass => {
+                        window_vibrancy::apply_acrylic(window, Some((0, 0, 0, 0)))
+                    }
+                    BackgroundMaterial::Frosted => {
+                        window_vibrancy::apply_acrylic(window, Some((0, 0, 0, 0)))
+                    }
+                    BackgroundMaterial::Solid => unreachable!(),
+                }
+                .map_err(|error| error.to_string())
+            });
+        match result {
+            Ok(()) => {
+                status.effective_material = material;
+                status.native_effect = true;
+            }
+            Err(_) => {
+                status.fallback_reason =
+                    Some("Native transparency is unavailable. Using a solid background.".into());
+            }
+        }
+    }
+    #[cfg(not(windows))]
+    {
+        let _ = window;
+        status.fallback_reason = Some(
+            if cfg!(target_os = "macos") {
+                "Native glass is unavailable in this macOS build. Using a solid background."
+            } else {
+                "Native glass is unavailable on this desktop. Using a solid background."
+            }
+            .into(),
+        );
+    }
+    status
+}
+
+#[cfg(windows)]
+fn native_backdrop_available() -> bool {
+    use windows_sys::Win32::UI::Accessibility::{HCF_HIGHCONTRASTON, HIGHCONTRASTW};
+    use windows_sys::Win32::{
+        Graphics::Dwm::DwmIsCompositionEnabled,
+        UI::WindowsAndMessaging::{SystemParametersInfoW, SPI_GETHIGHCONTRAST},
+    };
+    let mut composition = 0;
+    let mut contrast = HIGHCONTRASTW {
+        cbSize: std::mem::size_of::<HIGHCONTRASTW>() as u32,
+        dwFlags: 0,
+        lpszDefaultScheme: std::ptr::null_mut(),
+    };
+    // Failure to query accessibility/composition is treated conservatively.
+    unsafe {
+        if DwmIsCompositionEnabled(&mut composition) < 0
+            || composition == 0
+            || SystemParametersInfoW(
+                SPI_GETHIGHCONTRAST,
+                contrast.cbSize,
+                &mut contrast as *mut _ as *mut _,
+                0,
+            ) == 0
+            || contrast.dwFlags & HCF_HIGHCONTRASTON != 0
+        {
+            return false;
+        }
+    }
+    // The established crate checks the HRESULT of the public backdrop API on
+    // Windows 11 22H2+. Its older undocumented path cannot confirm success.
+    let mut version = windows_sys::Win32::System::SystemInformation::OSVERSIONINFOW {
+        dwOSVersionInfoSize: std::mem::size_of::<
+            windows_sys::Win32::System::SystemInformation::OSVERSIONINFOW,
+        >() as u32,
+        ..unsafe { std::mem::zeroed() }
+    };
+    unsafe {
+        windows_sys::Wdk::System::SystemServices::RtlGetVersion(&mut version) >= 0
+            && version.dwBuildNumber >= 22621
+    }
+}
diff --git a/apps/desktop/src-tauri/src/main.rs b/apps/desktop/src-tauri/src/main.rs
index 37bd4b7..44117f1 100644
--- a/apps/desktop/src-tauri/src/main.rs
+++ b/apps/desktop/src-tauri/src/main.rs
@@ -4,6 +4,7 @@
 compile_error!("Release desktop builds require --features custom-protocol to embed the frontend");
 
 mod ai_commands;
+mod appearance;
 mod data_directory;
 mod model_download;
 mod semantic_commands;
@@ -32,6 +33,7 @@ use tauri_plugin_global_shortcut::{GlobalShortcutExt, ShortcutState};
 use tauri_plugin_opener::OpenerExt;
 
 struct AppState {
+    appearance: Arc<Mutex<appearance::AppearanceStatus>>,
     ai: Arc<AiSession>,
     database: PathBuf,
     lexical: Arc<Mutex<SearchIndex>>,
@@ -85,6 +87,7 @@ fn measured_outcome(
 
 #[derive(Serialize)]
 struct DesktopStatus {
+    appearance: appearance::AppearanceStatus,
     settings: Settings,
     indexing: bool,
     shortcut_status: String,
@@ -94,6 +97,11 @@ struct DesktopStatus {
 #[tauri::command]
 fn desktop_status(state: State<'_, AppState>) -> Result<DesktopStatus, String> {
     Ok(DesktopStatus {
+        appearance: state
+            .appearance
+            .lock()
+            .map_err(|_| "Appearance is unavailable.")?
+            .clone(),
         settings: state
             .settings
             .lock()
@@ -124,6 +132,92 @@ fn set_cpu_threads(threads: usize, state: State<'_, AppState>) -> Result<Setting
     Ok(store.value.clone())
 }
 
+#[tauri::command]
+async fn set_appearance(
+    material: settings::BackgroundMaterial,
+    opacity: u8,
+    high_contrast: bool,
+    app: tauri::AppHandle,
+    state: State<'_, AppState>,
+) -> Result<appearance::AppearanceStatus, String> {
+    {
+        let mut store = state
+            .settings
+            .lock()
+            .map_err(|_| "Settings are unavailable.")?;
+        let mut next = store.value.clone();
+        next.background_material = material;
+        next.background_opacity = opacity;
+        store.save(next)?;
+    }
+    apply_saved_appearance(
+        app,
+        state.appearance.clone(),
+        material,
+        opacity,
+        high_contrast,
+    )
+    .await
+}
+
+#[tauri::command]
+async fn apply_appearance(
+    high_contrast: bool,
+    app: tauri::AppHandle,
+    state: State<'_, AppState>,
+) -> Result<appearance::AppearanceStatus, String> {
+    let (material, opacity) = {
+        let store = state
+            .settings
+            .lock()
+            .map_err(|_| "Settings are unavailable.")?;
+        (
+            store.value.background_material,
+            store.value.background_opacity,
+        )
+    };
+    apply_saved_appearance(
+        app,
+        state.appearance.clone(),
+        material,
+        opacity,
+        high_contrast,
+    )
+    .await
+}
+
+async fn apply_saved_appearance(
+    app: tauri::AppHandle,
+    current: Arc<Mutex<appearance::AppearanceStatus>>,
+    material: settings::BackgroundMaterial,
+    opacity: u8,
+    high_contrast: bool,
+) -> Result<appearance::AppearanceStatus, String> {
+    let window = app
+        .get_webview_window("main")
+        .ok_or("Main window is unavailable.")?;
+    let (sender, receiver) = std::sync::mpsc::sync_channel(1);
+    app.run_on_main_thread(move || {
+        let status = appearance::apply(&window, material, opacity, high_contrast);
+        let result = current
+            .lock()
+            .map_err(|_| "Appearance is unavailable.".to_string())
+            .map(|mut current| {
+                *current = status.clone();
+                status
+            });
+        let _ = sender.send(result);
+    })
+    .map_err(|error| error.to_string())?;
+    tauri::async_runtime::spawn_blocking(move || {
+        receiver
+            .recv_timeout(std::time::Duration::from_secs(5))
+            .map_err(|_| "Appearance update was cancelled.".to_string())?
+    })
+    .await
+    .map_err(|error| error.to_string())?
+}
+
 #[derive(Default, Deserialize)]
 struct Filters {
     #[serde(default)]
@@ -557,7 +651,10 @@ fn main() {
                     Err(error) => format!("Global shortcut unavailable: {error}"),
                 }
             };
+            let window = app.get_webview_window("main").ok_or_else(|| std::io::Error::other("Main window is unavailable."))?;
+            let appearance = appearance::apply(&window, settings.value.background_material, settings.value.background_opacity, false);
             app.manage(AppState {
+                appearance: Arc::new(Mutex::new(appearance)),
                 ai: Arc::new(AiSession::default()),
                 database,
                 lexical,
@@ -582,6 +679,8 @@ fn main() {
             prepare_media_preview,
             desktop_status,
             set_cpu_threads,
+            set_appearance,
+            apply_appearance,
             semantic_status,
             setup_model,
             index_embeddings,
diff --git a/apps/desktop/src-tauri/src/settings.rs b/apps/desktop/src-tauri/src/settings.rs
index 172b51e..3b2b56e 100644
--- a/apps/desktop/src-tauri/src/settings.rs
+++ b/apps/desktop/src-tauri/src/settings.rs
@@ -7,6 +7,19 @@ use std::{
     path::{Path, PathBuf},
 };
 
+#[derive(Clone, Copy, Debug, Default, Deserialize, Serialize, PartialEq, Eq)]
+#[serde(rename_all = "snake_case")]
+pub enum BackgroundMaterial {
+    Solid,
+    Glass,
+    #[default]
+    Frosted,
+}
+
+fn default_background_opacity() -> u8 {
+    90
+}
+
 const SETTINGS_VERSION: u32 = 2;
 const MAX_SETTINGS_BYTES: u64 = 128 * 1024;
 
@@ -14,6 +27,10 @@ const MAX_SETTINGS_BYTES: u64 = 128 * 1024;
 #[serde(deny_unknown_fields)]
 pub struct Settings {
     pub version: u32,
+    #[serde(default)]
+    pub background_material: BackgroundMaterial,
+    #[serde(default = "default_background_opacity")]
+    pub background_opacity: u8,
     pub roots: Vec<PathBuf>,
     pub cpu_threads: usize,
     /// Managed local Google EmbeddingGemma2 artifacts. Older settings default
@@ -26,6 +43,8 @@ impl Default for Settings {
     fn default() -> Self {
         Self {
             version: SETTINGS_VERSION,
+            background_material: BackgroundMaterial::default(),
+            background_opacity: default_background_opacity(),
             roots: Vec::new(),
             cpu_threads: 2,
             model_directory: None,
@@ -38,6 +57,9 @@ impl Settings {
         if self.version != SETTINGS_VERSION {
             return Err(format!("Unsupported settings version {}", self.version));
         }
+        if !(70..=100).contains(&self.background_opacity) {
+            return Err("Background opacity must be between 70 and 100 percent.".into());
+        }
         if !(1..=8).contains(&self.cpu_threads) {
             return Err("CPU threads must be between 1 and 8.".into());
         }
@@ -229,3 +251,31 @@ mod budget_migration_tests {
         assert_eq!(SettingsStore::load(path).unwrap().value.cpu_threads, 8);
     }
 }
+
+#[cfg(test)]
+mod appearance_tests {
+    use super::*;
+    #[test]
+    fn legacy_defaults_and_saved_appearance_round_trip() {
+        let directory = tempfile::tempdir().unwrap();
+        let path = directory.path().join("settings.json");
+        fs::write(&path, r#"{"version":2,"roots":[],"cpu_threads":2}"#).unwrap();
+        let mut store = SettingsStore::load(path.clone()).unwrap();
+        assert_eq!(store.value.background_material, BackgroundMaterial::Frosted);
+        assert_eq!(store.value.background_opacity, 90);
+        let mut next = store.value.clone();
+        next.background_material = BackgroundMaterial::Glass;
+        next.background_opacity = 70;
+        store.save(next.clone()).unwrap();
+        assert_eq!(SettingsStore::load(path.clone()).unwrap().value, next);
+        let bytes = fs::read(&path).unwrap();
+        for opacity in [0, 69, 101, 255] {
+            let mut invalid = next.clone();
+            invalid.background_opacity = opacity;
+            assert!(store.save(invalid).is_err());
+            assert_eq!(store.value, next);
+            assert_eq!(fs::read(&path).unwrap(), bytes);
+        }
+        assert!(serde_json::from_str::<BackgroundMaterial>("\"other\"").is_err());
+    }
+}
diff --git a/apps/desktop/src-tauri/tauri.windows.conf.json b/apps/desktop/src-tauri/tauri.windows.conf.json
new file mode 100644
index 0000000..5afb37a
--- /dev/null
+++ b/apps/desktop/src-tauri/tauri.windows.conf.json
@@ -0,0 +1 @@
+{"app":{"windows":[{"title":"searchi","width":1080,"height":760,"minWidth":680,"minHeight":540,"transparent":true}]}}
diff --git a/apps/desktop/src/appearance-state.ts b/apps/desktop/src/appearance-state.ts
new file mode 100644
index 0000000..db4d0d4
--- /dev/null
+++ b/apps/desktop/src/appearance-state.ts
@@ -0,0 +1,91 @@
+/** Only a confirmed native compositor effect may expose the desktop behind the shell.
+ * Reading surfaces retain their opaque backgrounds independently of this material. */
+export type BackgroundMaterial = "solid" | "glass" | "frosted";
+export interface AppearanceStatus {
+  requested_material: BackgroundMaterial;
+  effective_material: BackgroundMaterial;
+  opacity: number;
+  native_effect: boolean;
+  fallback_reason: string | null;
+}
+export function appearanceView(
+  status: AppearanceStatus | undefined,
+  highContrast: boolean,
+) {
+  const valid =
+    status &&
+    ["glass", "frosted"].includes(status.effective_material) &&
+    status.native_effect;
+  const material: BackgroundMaterial =
+    !highContrast && valid ? status.effective_material : "solid";
+  const opacity = Number.isFinite(status?.opacity)
+    ? Math.max(70, Math.min(100, status!.opacity))
+    : 90;
+  const message = highContrast
+    ? "High contrast uses a solid background. Your appearance preference is retained."
+    : status?.fallback_reason
+      ? `Solid background: ${status.fallback_reason}`
+      : material === "solid"
+        ? "Solid background. Search results and passages stay clear."
+        : "The desktop shows subtly through the window. Search results and passages stay clear.";
+  return { material, opacity, message };
+}
+
+/** Serializes compositor/settings mutations; a waiting update supersedes earlier
+ * pending work, retaining an unsaved preference when contrast also changes. */
+export class AppearanceUpdates {
+  revision = 0;
+  private running = false;
+  private pending: {
+    revision: number;
+    highContrast: boolean;
+    preference?: { material: BackgroundMaterial; opacity: number };
+  } | null = null;
+  private write: (update: {
+    highContrast: boolean;
+    preference?: { material: BackgroundMaterial; opacity: number };
+  }) => Promise<AppearanceStatus>;
+  private publish: (status: AppearanceStatus) => void;
+  private fail: (error: unknown) => void;
+  private settled: () => void;
+  constructor(
+    write: AppearanceUpdates["write"],
+    publish: AppearanceUpdates["publish"],
+    fail: AppearanceUpdates["fail"],
+    settled: AppearanceUpdates["settled"],
+  ) {
+    this.write = write;
+    this.publish = publish;
+    this.fail = fail;
+    this.settled = settled;
+  }
+  get busy() {
+    return this.running || this.pending !== null;
+  }
+  submit(
+    highContrast: boolean,
+    preference?: { material: BackgroundMaterial; opacity: number },
+  ) {
+    this.pending = {
+      revision: ++this.revision,
+      highContrast,
+      preference: preference ?? this.pending?.preference,
+    };
+    if (!this.running) void this.drain();
+  }
+  private async drain() {
+    this.running = true;
+    while (this.pending) {
+      const update = this.pending;
+      this.pending = null;
+      try {
+        const status = await this.write(update);
+        if (update.revision === this.revision) this.publish(status);
+      } catch (error) {
+        if (update.revision === this.revision) this.fail(error);
+      }
+    }
+    this.running = false;
+    this.settled();
+  }
+}
diff --git a/apps/desktop/src/main.ts b/apps/desktop/src/main.ts
index 74ff8cc..9db6679 100644
--- a/apps/desktop/src/main.ts
+++ b/apps/desktop/src/main.ts
@@ -6,6 +6,8 @@ import {
   isCaptureSurface,
   prepareMediaURL,
 } from "./desktop-api";
+import { appearanceView, AppearanceUpdates } from "./appearance-state";
+import type { AppearanceStatus, BackgroundMaterial } from "./appearance-state";
 import { listen } from "@tauri-apps/api/event";
 import {
   AutomaticIndexGate,
@@ -84,11 +86,14 @@ interface Settings {
   version: number;
   roots: string[];
   cpu_threads: number;
+  background_material?: BackgroundMaterial;
+  background_opacity?: number;
 }
 interface DesktopStatus {
   settings: Settings;
   indexing: boolean;
   shortcut_status: string;
+  appearance?: AppearanceStatus;
   watching?: {
     root: string;
     attached: boolean;
@@ -140,7 +145,7 @@ interface SearchRequest {
 }
 
 document.querySelector<HTMLDivElement>("#app")!.innerHTML = `
-<header><div class="brand"><img class="brand-symbol" src="/searchi-symbol.png" alt=""><span class="wordmark">searchi</span></div><span id="index-status" class="index-status">Loading your index…</span><div class="header-actions"><button id="add-folder" class="quiet-button" type="button">${icon("folderAdd")}<span>Add folder</span></button><details id="filters" class="popover"><summary>${icon("filter")}<span id="scope-label">All files</span>${icon("chevron")}</summary><div class="popover-panel"><h2>Filter your search</h2><div class="filters-grid"><label>Search scope<select id="filter-scope"><option value="all">All files</option><option value="code">Code</option></select></label><label>Within folder<input id="filter-folder" placeholder="Absolute folder path"></label><label>Extension<input id="filter-extension" placeholder="pdf, md, txt…"></label><label>Minimum size (bytes)<input id="filter-min" type="number" min="0" step="1"></label><label>Maximum size (bytes)<input id="filter-max" type="number" min="0" step="1"></label><label>Minimum duration (seconds)<input id="filter-duration-min" type="number" min="0" step="0.1"></label><label>Maximum duration (seconds)<input id="filter-duration-max" type="number" min="0" step="0.1"></label><label>Modified from<input id="filter-after" type="date"></label><label>Modified through<input id="filter-before" type="date"></label></div></div></details><details id="settings" class="popover"><summary class="icon-button" aria-label="Settings" title="Settings">${icon("settings")}</summary><div class="popover-panel"><h2>Folders & resources</h2><p id="shortcut-status" class="hint"></p><label class="resource-label" for="cpu-threads">CPU thread budget</label><select id="cpu-threads">${Array.from({ length: 8 }, (_, i) => `<option value="${i + 1}">${i + 1} ${i === 1 ? "threads (default)" : i ? "threads" : "thread (low power)"}</option>`).join("")}</select><p class="hint">Search automatically uses supported acceleration with CPU fallback. Two or more threads let search run alongside indexing; one thread can delay semantic search during indexing.</p><ul id="roots" aria-label="Selected folders"></ul><label class="performance-toggle"><input id="performance-toggle" type="checkbox">Performance details</label><p class="hint">Local timing diagnostics for new searches. Queries and file paths are excluded from copied reports.</p><details id="ai-settings" class="ai-settings"><summary>Optional AI answers</summary><label class="performance-toggle"><input id="ai-enabled" type="checkbox">Enable summaries and questions</label><p class="hint">Disabled by default. Only an explicit action sends the shown passage to your configured endpoint.</p><label>OpenAI-compatible base URL<input id="ai-endpoint" type="url" placeholder="http://localhost:1234/v1" autocomplete="off" maxlength="2048"></label><label>Model<input id="ai-model" placeholder="Your model name" autocomplete="off" maxlength="200"></label><label>API key (optional, this session only)<input id="ai-key" type="password" autocomplete="off" maxlength="2048"></label><button id="ai-clear-key" type="button">Clear key</button><p class="hint">Endpoint, model and key stay in this app session. The key authenticates requests to your endpoint.</p></details></div></details></div></header>
+<header><div class="brand"><img class="brand-symbol" src="/searchi-symbol.png" alt=""><span class="wordmark">searchi</span></div><span id="index-status" class="index-status">Loading your index…</span><div class="header-actions"><button id="add-folder" class="quiet-button" type="button">${icon("folderAdd")}<span>Add folder</span></button><details id="filters" class="popover"><summary>${icon("filter")}<span id="scope-label">All files</span>${icon("chevron")}</summary><div class="popover-panel"><h2>Filter your search</h2><div class="filters-grid"><label>Search scope<select id="filter-scope"><option value="all">All files</option><option value="code">Code</option></select></label><label>Within folder<input id="filter-folder" placeholder="Absolute folder path"></label><label>Extension<input id="filter-extension" placeholder="pdf, md, txt…"></label><label>Minimum size (bytes)<input id="filter-min" type="number" min="0" step="1"></label><label>Maximum size (bytes)<input id="filter-max" type="number" min="0" step="1"></label><label>Minimum duration (seconds)<input id="filter-duration-min" type="number" min="0" step="0.1"></label><label>Maximum duration (seconds)<input id="filter-duration-max" type="number" min="0" step="0.1"></label><label>Modified from<input id="filter-after" type="date"></label><label>Modified through<input id="filter-before" type="date"></label></div></div></details><details id="settings" class="popover"><summary class="icon-button" aria-label="Settings" title="Settings">${icon("settings")}</summary><div class="popover-panel"><h2>Settings</h2><section class="appearance-settings" aria-labelledby="appearance-heading"><h3 id="appearance-heading">Appearance</h3><label for="background-material">Window background</label><select id="background-material"><option value="solid">Solid</option><option value="glass">Glass</option><option value="frosted">Frosted glass</option></select><div class="appearance-opacity"><label for="background-opacity">Background tint</label><output id="background-opacity-value" for="background-opacity">90%</output></div><input id="background-opacity" type="range" min="70" max="100" step="1" value="90" aria-describedby="appearance-status"><p id="appearance-status" class="hint" role="status">A subtle window background. Search results and passages stay clear.</p></section><h3>Folders & resources</h3><p id="shortcut-status" class="hint"></p><label class="resource-label" for="cpu-threads">CPU thread budget</label><select id="cpu-threads">${Array.from({ length: 8 }, (_, i) => `<option value="${i + 1}">${i + 1} ${i === 1 ? "threads (default)" : i ? "threads" : "thread (low power)"}</option>`).join("")}</select><p class="hint">Search automatically uses supported acceleration with CPU fallback. Two or more threads let search run alongside indexing; one thread can delay semantic search during indexing.</p><ul id="roots" aria-label="Selected folders"></ul><label class="performance-toggle"><input id="performance-toggle" type="checkbox">Performance details</label><p class="hint">Local timing diagnostics for new searches. Queries and file paths are excluded from copied reports.</p><details id="ai-settings" class="ai-settings"><summary>Optional AI answers</summary><label class="performance-toggle"><input id="ai-enabled" type="checkbox">Enable summaries and questions</label><p class="hint">Disabled by default. Only an explicit action sends the shown passage to your configured endpoint.</p><label>OpenAI-compatible base URL<input id="ai-endpoint" type="url" placeholder="http://localhost:1234/v1" autocomplete="off" maxlength="2048"></label><label>Model<input id="ai-model" placeholder="Your model name" autocomplete="off" maxlength="200"></label><label>API key (optional, this session only)<input id="ai-key" type="password" autocomplete="off" maxlength="2048"></label><button id="ai-clear-key" type="button">Clear key</button><p class="hint">Endpoint, model and key stay in this app session. The key authenticates requests to your endpoint.</p></details></div></details></div></header>
 <main>
 <form id="search-form" role="search"><label class="sr-only" for="query">Search your files</label>${icon("search")}<input id="query" type="search" placeholder="Search a filename, phrase, or what you remember…" autocomplete="off" spellcheck="false" maxlength="1000"><kbd id="focus-shortcut">Ctrl K</kbd><button id="clear-query" class="icon-button" type="button" aria-label="Clear search" title="Clear search">${icon("dismiss")}</button></form>
 <details id="semantic-panel"><summary>${icon("sparkles")}<span id="semantic-status" role="status" aria-live="polite">Preparing local semantic search…</span><span id="semantic-device" class="kind">Local model</span>${icon("chevron")}</summary><div class="semantic-content"><strong>Google EmbeddingGemma 2</strong><progress id="semantic-progress" aria-label="Local model setup progress"></progress><p id="semantic-detail" class="hint">The verified local model downloads automatically on first launch (865 MB). Your files stay on this device.</p><div class="semantic-actions"><button id="retry-model" type="button" hidden>Retry model setup</button><button id="rebuild-semantic" type="button" hidden>Rebuild semantic index</button></div></div></details>
@@ -465,8 +470,88 @@ async function refreshSemanticStatus() {
   }
 }
 
-async function refreshDesktopStatus() {
+const highContrast = window.matchMedia("(forced-colors: active)");
+const appearanceUpdates = new AppearanceUpdates(
+  (update) =>
+    invoke<AppearanceStatus>(
+      update.preference ? "set_appearance" : "apply_appearance",
+      { ...update.preference, highContrast: update.highContrast },
+    ),
+  (status) => renderAppearance(status),
+  showError,
+  () => {
+    renderAppearance();
+    void refreshDesktopStatus(true).catch(showError);
+  },
+);
+let lastAppearance: AppearanceStatus | undefined;
+function renderAppearance(
+  status?: AppearanceStatus,
+  settings?: Settings,
+  restoreControls = false,
+) {
+  if (status) lastAppearance = status;
+  const material = element<HTMLSelectElement>("background-material");
+  const opacity = element<HTMLInputElement>("background-opacity");
+  if (
+    !appearanceUpdates.busy &&
+    settings &&
+    (restoreControls || document.activeElement !== opacity)
+  ) {
+    material.value = settings.background_material ?? "frosted";
+    opacity.value = String(settings.background_opacity ?? 90);
+  }
+  const view = appearanceView(lastAppearance, highContrast.matches);
+  document.documentElement.dataset.backgroundMaterial = view.material;
+  document.documentElement.style.setProperty(
+    "--window-opacity",
+    String(view.opacity / 100),
+  );
+  element("background-opacity-value").textContent = `${opacity.value}%`;
+  material.disabled = !isNativeDesktop() || appearanceUpdates.busy;
+  opacity.disabled =
+    !isNativeDesktop() ||
+    appearanceUpdates.busy ||
+    material.value === "solid" ||
+    highContrast.matches;
+  element("appearance-status").textContent = view.message;
+}
+function saveAppearance() {
+  if (!isNativeDesktop()) return;
+  appearanceUpdates.submit(highContrast.matches, {
+    material: element<HTMLSelectElement>("background-material")
+      .value as BackgroundMaterial,
+    opacity: Number(element<HTMLInputElement>("background-opacity").value),
+  });
+  renderAppearance();
+}
+element("background-material").addEventListener("change", () => {
+  void saveAppearance();
+});
+element("background-opacity").addEventListener("input", () => {
+  element("background-opacity-value").textContent =
+    `${element<HTMLInputElement>("background-opacity").value}%`;
+});
+element("background-opacity").addEventListener("change", () => {
+  void saveAppearance();
+});
+highContrast.addEventListener("change", () => {
+  renderAppearance(); // Fail closed immediately, before the native effect changes.
+  if (isNativeDesktop()) appearanceUpdates.submit(highContrast.matches);
+});
+
+async function refreshDesktopStatus(restoreAppearanceControls = false) {
+  const appearanceRevision = appearanceUpdates.revision;
   const status = await invoke<DesktopStatus>("desktop_status");
+  if (
+    !appearanceUpdates.busy &&
+    appearanceRevision === appearanceUpdates.revision
+  )
+    renderAppearance(
+      status.appearance,
+      status.settings,
+      restoreAppearanceControls,
+    );
   indexingActive = status.indexing;
   element("shortcut-status").textContent = status.shortcut_status;
   element("focus-shortcut").textContent = status.shortcut_status.includes(
@@ -1433,6 +1518,10 @@ document.addEventListener("keydown", (event) => {
 });
 async function initializeSurface() {
   await initializeDesktopSurface();
+  renderAppearance();
+  if (isNativeDesktop()) {
+    appearanceUpdates.submit(highContrast.matches);
+  }
   if (isDesktopSurface()) {
     if (isCaptureSurface()) {
       for (const id of [
diff --git a/apps/desktop/src/style.css b/apps/desktop/src/style.css
index 60e69a1..5d26540 100644
--- a/apps/desktop/src/style.css
+++ b/apps/desktop/src/style.css
@@ -949,3 +949,86 @@ aside pre.code-passage {
 aside pre.code-passage {
   line-height: 1.6;
 }
+
+/* Native effects blur the desktop; CSS only supplies its readable shell tint.
+   Never use backdrop-filter as a substitute for an operating-system material. */
+:root[data-background-material="glass"],
+:root[data-background-material="frosted"] {
+  background: transparent;
+}
+:root[data-background-material="glass"] #app {
+  background: rgba(248, 250, 252, var(--window-opacity, 0.9));
+}
+:root[data-background-material="frosted"] #app {
+  background: rgba(
+    248,
+    250,
+    252,
+    calc(0.35 + var(--window-opacity, 0.9) * 0.65)
+  );
+}
+.appearance-settings {
+  padding-bottom: 16px;
+  margin-bottom: 18px;
+  border-bottom: 1px solid var(--line);
+}
+.popover-panel h3 {
+  margin: 0 0 12px;
+  font-size: 13px;
+}
+.appearance-settings > label,
+.appearance-opacity label {
+  font-size: 12px;
+  color: var(--muted);
+}
+#background-material {
+  display: block;
+  width: 100%;
+  margin: 7px 0 15px;
+}
+.appearance-opacity {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 10px;
+}
+#background-opacity-value {
+  font-size: 12px;
+  font-variant-numeric: tabular-nums;
+}
+#background-opacity {
+  display: block;
+  width: 100%;
+  padding: 0;
+  margin: 9px 0;
+  accent-color: var(--brand);
+}
+@media (prefers-color-scheme: dark) {
+  :root[data-background-material="glass"] #app {
+    background: rgba(11, 15, 20, var(--window-opacity, 0.9));
+  }
+  :root[data-background-material="frosted"] #app {
+    background: rgba(
+      11,
+      15,
+      20,
+      calc(0.35 + var(--window-opacity, 0.9) * 0.65)
+    );
+  }
+}
+@media (forced-colors: active) {
+  :root,
+  :root[data-background-material] #app {
+    background: Canvas;
+  }
+}
+
+/* Strong shell copy remains legible even above an unusually bright/dark desktop.
+   Opaque reading panes retain the quieter secondary-text hierarchy. */
+:root:not([data-background-material="solid"])[data-background-material] header,
+:root:not([data-background-material="solid"])[data-background-material] footer,
+:root:not([data-background-material="solid"])[data-background-material]
+  #semantic-panel
+  > summary {
+  --muted: var(--ink);
+}
diff --git a/apps/desktop/tests/appearance-state.test.mjs b/apps/desktop/tests/appearance-state.test.mjs
new file mode 100644
index 0000000..ca1a12d
--- /dev/null
+++ b/apps/desktop/tests/appearance-state.test.mjs
@@ -0,0 +1,67 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import {appearanceView, AppearanceUpdates} from '../src/appearance-state.ts';
+const glass = () => ({requested_material:'frosted', effective_material:'frosted', opacity:90, native_effect:true, fallback_reason:null});
+test('transparency requires a confirmed native effect', () => {
+  assert.equal(appearanceView(undefined, false).material, 'solid');
+  assert.equal(appearanceView({...glass(), native_effect:false}, false).material, 'solid');
+  assert.equal(appearanceView(glass(), false).material, 'frosted');
+  assert.equal(appearanceView({...glass(), effective_material:'glass'}, false).material, 'glass');
+});
+test('high contrast stays opaque without losing the preference', () => {
+  const status = glass();
+  assert.equal(appearanceView(status, true).material, 'solid');
+  assert.match(appearanceView(status, true).message, /preference is retained/);
+  assert.equal(status.requested_material, 'frosted');
+  assert.equal(appearanceView(status, false).material, 'frosted');
+});
+test('fallback is honest and shell tint is bounded', () => {
+  const fallback = {...glass(), effective_material:'solid', native_effect:false, fallback_reason:'Native effects unavailable.'};
+  assert.match(appearanceView(fallback, false).message, /Solid background/);
+  assert.equal(appearanceView({...glass(), opacity:5}, false).opacity, 70);
+  assert.equal(appearanceView({...glass(), opacity:110}, false).opacity, 100);
+  assert.equal(appearanceView({...glass(), opacity:NaN}, false).opacity, 90);
+});
+
+test('rapid pending changes serialize and only publish the latest response', async () => {
+  const writes = [], published = [], errors = [];
+  let resolve;
+  let settled;
+  const done = new Promise(r => settled = r);
+  const queue = new AppearanceUpdates(update => {
+    writes.push(update);
+    if (writes.length === 1) return new Promise(r => resolve = r);
+    return Promise.resolve({...glass(), opacity:update.preference.opacity});
+  }, status => published.push(status), error => errors.push(error), settled);
+  queue.submit(false, {material:'glass', opacity:80});
+  queue.submit(false, {material:'frosted', opacity:95});
+  queue.submit(true); // Preserve the pending saved preference when contrast toggles.
+  assert.equal(writes.length, 1);
+  assert.equal(queue.busy, true);
+  resolve({...glass(), effective_material:'glass', opacity:80});
+  await done;
+  assert.equal(writes.length, 2);
+  assert.equal(writes[1].highContrast, true);
+  assert.deepEqual(writes[1].preference, {material:'frosted', opacity:95});
+  assert.equal(published.length, 1);
+  assert.equal(published[0].opacity, 95);
+  assert.equal(queue.busy, false);
+  assert.deepEqual(errors, []);
+});
+test('failed stale mutation cannot suppress a queued contrast restore', async () => {
+  let reject, settled;
+  const done = new Promise(r => settled = r);
+  const writes = [], errors = [], published = [];
+  const queue = new AppearanceUpdates(update => {
+    writes.push(update);
+    return writes.length === 1 ? new Promise((_,r) => reject=r) : Promise.resolve(glass());
+  }, status => published.push(status), error => errors.push(error), settled);
+  queue.submit(true);
+  queue.submit(false);
+  reject(new Error('owned stale failure'));
+  await done;
+  assert.equal(writes.length,2);
+  assert.equal(writes[1].highContrast,false);
+  assert.equal(published.length,1);
+  assert.deepEqual(errors,[]);
+});
diff --git a/docs/BRAND.md b/docs/BRAND.md
index 0eb52cf..6647ceb 100644
--- a/docs/BRAND.md
+++ b/docs/BRAND.md
@@ -44,3 +44,19 @@ The brand board's example `searchi winter.pdf` is illustrative. Documentation
 must show the actual supported CLI syntax. Model downloads require networking;
 file extraction, inference, and search remain local. Never turn visual marketing
 copy into unsupported performance or production-readiness claims.
+
+## Window materials
+
+Appearance settings offer Solid, Glass and Frosted glass, with a bounded
+70–100% tint-strength control (90% by default). Frosted glass layers an extra
+35% milk tint beneath that control; Glass has the lighter direct tint. Both
+currently share Windows native acrylic blur. Native compositor effects expose and soften
+the actual desktop; CSS transparency alone cannot blur other windows. If the
+platform cannot apply its native effect, use an honest opaque fallback. Forced
+colors always make the shell opaque while retaining the saved preference.
+Search input, results, previews and settings keep their opaque reading surfaces;
+never lower text opacity or introduce decorative wallpaper simulations.
+Native acrylic currently requires Windows 11 22H2 or later with a compatible
+compositor and system transparency settings. macOS, Linux and older Windows
+currently use opaque fallback. Native material appearance and compositor
+performance still need actual OS validation.
diff --git a/docs/ROADMAP.md b/docs/ROADMAP.md
index bf5278b..6220f7f 100644
--- a/docs/ROADMAP.md
+++ b/docs/ROADMAP.md
@@ -36,6 +36,10 @@ Status: implementation started; no production release yet. Windows VM tests use
   see [incremental semantic design](INCREMENTAL-SEMANTIC.md).
 
 ## 3. Desktop
+- [x] Persisted solid/glass/frosted appearance and bounded background tint;
+  opaque reading surfaces, serialized settings updates and high-contrast fallback.
+- [ ] Verify native glass visually on Windows 11 22H2+; macOS/Linux currently
+  use an explicit opaque fallback. Native compositor performance remains open.
 - [ ] Tauri shell, accessible keyboard search, progressive results and previews (shell built; UI interaction verification pending).
 - [ ] Folder selection, exclusion controls, resource budgets and indexing status.
 - [ ] Global shortcut, opening/revealing source files, PDF page navigation.
@@ -197,8 +201,10 @@ Audit and measured priorities: [EMBEDDINGGEMMA2-AUDIT.md](EMBEDDINGGEMMA2-AUDIT.
       media retry status across engine/desktop; genuine CPU promotion reuses
       identical document vectors with zero document re-encodes.
       See [early text setup](EARLY-TEXT-SETUP.md).
-    - [ ] Verify native first use during genuine delayed media download,
-      failure/retry and promotion; record actual first-input timing separately.
+    - [x] Native Linux CPU first use during genuine delayed media download,
+      failure/retry and promotion; actual first-input timing recorded separately.
+      Focused activation retest preserves selection and query process, records
+      fresh timing/current generation after idle. Windows/macOS checks stay open.
 - [ ] Buffered bounded media IPC and linear UTF-8 source-window offset processing.
 - [ ] Evaluate 256d MRL and content-specific vision budgets on independent data.
 - [ ] Validate GPU precision, synchronization-aware timings, parity and fallback
diff --git a/docs/WINDOW-MATERIALS.md b/docs/WINDOW-MATERIALS.md
new file mode 100644
index 0000000..120366d
--- /dev/null
+++ b/docs/WINDOW-MATERIALS.md
@@ -0,0 +1,42 @@
+# Configurable window backgrounds
+
+Settings → Appearance offers Solid, Glass and Frosted glass. Preferences are
+saved locally. The tint-strength control ranges from 70 to 100%; the default
+is Frosted glass at 90%. Glass uses the direct tint; Frosted glass adds a 35%
+milk layer beneath it. This changes background treatment, never text opacity.
+
+Search input, result and preview panes and settings retain opaque reading
+surfaces. Shell secondary text becomes stronger over translucent backgrounds.
+High contrast immediately forces an opaque surface without changing the saved
+preference. Appearance writes are serialized; obsolete replies cannot overwrite
+the latest preference or accessibility state.
+
+## Platform support
+
+Windows 11 22H2 and newer use a transparent WebView and the established
+window-vibrancy crate's native acrylic implementation. Glass and Frosted glass
+share that native blur; their frontend milk tint differs. Desktop composition
+and OS high-contrast checks precede application, and native HRESULT failure
+returns an explicit solid fallback. Native window chrome remains enabled.
+
+Older Windows, macOS and Linux currently use a solid fallback. This macOS build
+does not enable private WebView transparency APIs. CSS backdrop-filter is not
+used to pretend the desktop is blurred. The settings report the effective
+material and explain the fallback.
+
+## Recorded checks, 2026-10-13
+
+- 57 frontend tests passed, including missing native capability, high contrast,
+  bounded tint, serialized pending preferences and stale failure handling.
+- Frontend type checking, production build and formatting passed.
+- 33 desktop Rust tests passed with one test thread; settings checks cover legacy
+  defaults, persisted preferences and invalid-value rejection preserving the
+  previous file and memory. An earlier parallel run hit a transient Windows
+  AccessDenied in an existing atomic-settings replacement test.
+- Desktop Rust formatting and strict all-target Clippy passed.
+
+Actual native material appearance and resize/compositor performance remain
+unverified. Enabled browser inventory was empty; native Windows capture/input
+access was unavailable. These checks establish implementation and persistence,
+not visual quality on release hardware. macOS/Linux native materials remain
+future work.