Redoing the dark theme to be more plain CSS and not rely on central variables

This is a fix for #8420.

This turns the dark theme into a reasonably simple collection of rules that do not rely on any of the built-in LESS files.

cc: @MiguelCastillo @larz0 @peterflynn 
Ha! Actually, it does require a newline. And that's the cause of #8481.