Refactor incognito recoloring + support HighContrast
Adwaita has some special casing for default color variables. Since we were using a bunch of different variables, this led to some styles being a bit wrong, e.g. buttons having too light borders. Instead, override the default variables.
Make the style reusable so it can be applied for automation mode the same way without copypasta:
Since HighContrast is now just a variant of Adwaita (and HighContrastInverse is a variant of Adwaita:dark), it's trivial to also support them too:
Edited by Alice Mikhaylenko