[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
Step editor background color in dark theme is same as light theme (white) -- Should be medium/dark gray in dark theme. May want to use a light gray in light theme.