The theme selector should select the last match to when opening to keep
the current theme active but it should select the first when searching
to pick the best match.

fixes #28081


https://github.com/user-attachments/assets/b46b9742-4715-4c7a-8f17-2c19a8668333

Release Notes:

- Fixed selecting the correct theme when searching

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Co-authored-by: Peter Tripp <peter@zed.dev>