Use dropdown.background color in debug dropdown

[jeanp413]

This PR fixes #95704

Not sure if it's still required to fallback to SIDE_BAR_BACKGROUND for the background is there is no color customization for dropdown.background
Now it looks like this when there is no color customization

Also removed some css rules (added in #95935) that were not needed.

[isidorn]

@jeanp413 thanks a lot for this PR. Though this week we are in endgame, thus we are testing and no new features. So I will review and merge this next week, assigning to May.

[isidorn]

Works lovely! Thanks a lot for this PR. Merging in ☀️

[isidorn]

Actually there are issues on Mac which I did not originally see. So just the default dark theme
Notice the black lines
This widget should ideally be continues the the seperation between the play button and the rest should not be distinguishable.
@jeanp413 let me know if you can look into this. Thanks!

[jeanp413]

Sure I'll look into it.

[isidorn]

Great, thanks!