(REVIEW ONLY) Redoing the dark theme to be more plain CSS and not rely on central variables

This is a fix for #8420. **It's not quite ready to land because the inline editors don't look right.**

This turns the dark theme into a reasonably simple collection of rules that do not rely on any of the built-in LESS files.

I'm not sure right now what's missing for the inline editors and don't have time to look into it now. I wanted to get this up for review, though.

cc: @MiguelCastillo @larz0 @peterflynn