- clean out unused colors; add desaturated light color
- remove long unused drawing functions; add high contrast color mixin
- de-duplicate and move common colors to a default-colors stylesheet
- rework how HighContrast is defined; clean up HC cruft
- bring over named color defines and other colors from libadwaita
- change how panel colors are defined
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3097>