Closes #5049.

When the system is in light mode but Zed uses a dark theme, the macOS
window chrome — the window border and the titlebar — is rendered light,
leaving a pale, low-contrast edge that makes the window look washed out.
Zed's own content renders correctly; the affected chrome is drawn by
AppKit, not Zed.

Why it happens: `NSWindow` conforms to `NSAppearanceCustomization`, so a
window's appearance is inherited app → window → view and resolved
through `effectiveAppearance` unless an explicit appearance is set. Zed
never set one, so every window inherited the *system* appearance and
AppKit drew the chrome for light mode regardless of the dark theme.

The fix uses the override AppKit provides for exactly this: set
`NSApplication.appearance` app-wide, so every window inherits it. VSCode
(`window.systemColorTheme`) and JetBrains IDEs do the same.

<img width="2600" height="1346" alt="after-light"
src="https://github.com/user-attachments/assets/c31376af-8f51-4c36-a62f-55714f63b59c"
/>

## GPUI

- Add `App::set_window_appearance(Option<WindowAppearance>)`, the setter
paired with the existing `App::window_appearance()` getter. `Some(_)`
forces a light/dark appearance; `None` clears the override and follows
the system again.
- macOS sets `NSApplication.appearance` (`None` → `nil`); no-op on other
platforms.
- Demo: `cargo run -p gpui --example window_appearance`.

## Zed

- An explicit light/dark theme (or a static theme) forces the matching
chrome; `System` follows the OS.
- Wired once at startup via a global settings observer, so every window
— including the settings window — stays in sync.

| Before | After |
| --- | --- |
| <img width="3630" height="1878" alt="before-light"
src="https://github.com/user-attachments/assets/44c9ab66-c0cf-433a-93b0-17446b34a284"
/> | <img width="3686" height="1832" alt="SCR-20260609-nzjk"
src="https://github.com/user-attachments/assets/32b0412b-0913-4e68-acba-bf39dcb6aa6a"
/> |


References:
[`NSAppearanceCustomization`](https://developer.apple.com/documentation/appkit/nsappearancecustomization)
·
[`NSApplication.appearance`](https://developer.apple.com/documentation/appkit/nsapplication/appearance)

Release Notes:

- Fixed the macOS window border and titlebar not matching the selected
theme when the system appearance differed (for example, a dark theme
under a light system theme).
([#5049](https://github.com/zed-industries/zed/issues/5049))

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>