Minor tweaks to playground color scheme (#5754)

## Summary

I kind of hate the light mode theme, but they now use colors from our
actual palette:

<img width="1792" alt="Screen Shot 2023-07-13 at 10 15 14 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/f1da0153-d6ed-4b65-9419-b824f2cad614">
<img width="1792" alt="Screen Shot 2023-07-13 at 10 15 12 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/d9452e10-796b-4b7f-bf3f-7af6e0b14fc0">
<img width="1792" alt="Screen Shot 2023-07-13 at 10 15 10 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/f75e7c1c-3b5a-4a78-8bb8-d8b4d40a337d">
<img width="1792" alt="Screen Shot 2023-07-13 at 10 15 07 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/52c23108-b9c2-4a1f-adf0-e11098dbdc5d">