Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #54607

Release Notes:

- Settings UI theme conversion now respects system appearance.
When switching Theme Mode from Dynamic (System) to Static, Zed now picks
the static theme that matches the current OS app appearance (light or
dark) instead of always defaulting to the dark variant.

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Co-authored-by: Lukas Wirth <lukas@zed.dev>