[Bug] dark: utilities follow the OS color scheme, not the selected theme (missing Tailwind v4 @custom-variant binding) #2848

Describe the bug

The desktop app's theming is class-driven: ThemeProvider resolves the active
theme and sets .dark / .light on <html>
(desktop/src/shared/theme/ThemeProvider.tsx), and the synchronous bootstrap in
desktop/index.html seeds that class before first paint.

However, the app is on Tailwind v4, where the dark: variant defaults to
the prefers-color-scheme media query unless it is explicitly rebound to a
class. Neither desktop/src/shared/styles/globals.css nor the legacy config
loaded via @config declares such a binding (tailwind.config.js has no
darkMode key, and there is no @custom-variant anywhere in the repo).

The result: all 127 dark: utilities across 48 desktop files respond to the
OS appearance, while every CSS-variable-driven style responds to the
selected theme. Any user who picks an explicit theme that disagrees with
their OS appearance gets mixed styling.

To reproduce

1. macOS in dark appearance.
2. In Buzz: Settings → Appearance → mode Light, any light theme
   (e.g. Catppuccin Latte).
3. Look at surfaces styled with dark: utilities, e.g. the message composer
   (desktop/src/features/messages/ui/MessageComposer.tsx uses
   dark:bg-background/70 dark:backdrop-blur-xl ...): they apply their
   dark-mode treatment on top of the light theme.
4. The inverse also holds (OS light + explicit dark theme): dark: styling
   silently never applies.

Evidence (compiled CSS)

Compiling desktop/src/shared/styles/globals.css with the repo's own config
(Tailwind CLI 4.3.x) emits every dark: utility inside a media query — there
is no .dark-class selector at all:

@media (prefers-color-scheme: dark) {
  .dark\:border-blue-500\/30 { ... }
  .dark\:bg-background\/70 { ... }
  /* ...every dark: utility in the app... */
}

Expected behavior

dark: utilities should follow the theme class the app already maintains, i.e.
compile to .dark\:bg-background\/70:where(.dark, .dark *) { ... }.

Why it goes mostly unnoticed

The default is "Buzz + Follow system", so theme and OS agree for most users.
Only users who pin a theme that disagrees with their OS appearance see the
mismatch — which may also be a contributing factor in other light-mode
visibility reports.

Fix

One line in each Tailwind entry point (desktop and web, which has the same
latent pattern — its ThemeProvider also sets the class):

@custom-variant dark (&:where(.dark, .dark *));

Verified by recompiling: zero prefers-color-scheme occurrences remain and all
dark: utilities bind to the theme class. PR incoming.

Environment

• Found by static analysis + compiling the stylesheet with the repo's config
  (Tailwind CLI 4.3.3); reproducible on any platform.