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.

src/agent/changes.rs

@@ -150,7 +150,7 @@ impl RenderOnce for FileChangeCard {
                     })
                     .child(Disclosure::new((self.id.clone(), "chevron"), open).size(IconSize::Sm))
                     .child(
-                        Icon::new(file_icon(&self.change.path))
+                        Icon::new(file_icon(&self.change.path, cx))
                             .size(IconSize::Sm)
                             .color(colors.fg_muted),
                     )