Instead of a dropdown, it is now just a one-click icon.

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

Release Notes:

- Improved theme toggle for the documentation website

---------

Co-authored-by: Gaauwe Rombouts <mail@grombouts.nl>