Tweak background on theme button (#6290)
## Summary
It's now white on-hover as opposed to yellow, to match the copy button:
<img width="1792" alt="Screen Shot 2023-08-02 at 8 52 10 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/96d5cbf9-ef33-4fba-8888-f2a4af9a6ec4">
