[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, I never noticed this before, but in the radio buttons (RGBa|Hex|HSLa) the selected state looks unselected -- so it looks like 2 selected buttons and 1 unselected button. It's not as noticeable when control has focus because selected button has glow outline, so click on another control and look at unfocused state.