Add configurable native glass window backgrounds

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.