[REVIEW ONLY] Dark UI theme for inline editors

This is a spinoff of @larz0's work in #8362 with a goal of fixing #8379.

As of this initial push, I have done inline CSS and color editors. The timing editor is almost done, and quick docs remains.

cc @MiguelCastillo
In the light theme, the bg color of the Quick Docs viewer contrasts with the bg color of the editor, so the difference between the 2 elements is very clear. In the dark theme the 2 bg colors are almost the same, so it's hard to see any difference.