Theme · Platform: one platform for buttons and caps, icon themes, frosted panes

theme::Platform moves out of typography and shell's ControlsStyle folds
into it; the theme's platform starts as the system's. Window buttons Ely
draws, the title bar's layout and every key cap follow it, secondary
included, through typography::keys::keystroke; a title bar whose buttons
the OS draws follows the OS. primitives::IconTheme lays an owner's rules
by file name and extension over Ely's map, for every file icon.
shell::Vibrancy fills a pane with the palette's new glass, translucent
so a window opened Blurred shows its backdrop, solid under high contrast.
gpui 0.2.2 has no Mica; that line stays blocked.

tasks/ch31-43.md

@@ -266,9 +266,10 @@
 
 ## T39c Theme · Platform
 
-- [ ] IconThemeProvider
-- [ ] Platform Style
-- [ ] VibrancyBackground / Mica / Acrylic — native `WindowBackgroundAppearance::Blurred`
+- [x] IconThemeProvider → `primitives::IconTheme`, an owner's rules by name and extension over `file_icon`
+- [x] Platform Style → `theme::Platform`, the theme's `platform`: the window buttons Ely draws, the title bar, key caps
+- [x] VibrancyBackground / Acrylic → `shell::Vibrancy` in a window opened `WindowBackgroundAppearance::Blurred`
+- [ ] Mica — blocked: gpui 0.2.2's one blur, `Blurred`, is acrylic on Windows
 
 ## T40 i18n & a11y — `src/i18n`