[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
The Dark Theme changes the color (tone?) of the transparent white/gray checkerboard. I think this should be the same regardless of Theme.