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 
@dangoor Should we file a (post-0.42) bug on the Themes code being so finicky about trailing newlines?  I could see third-party theme authors being totally befuddled by this as well...