Merged requested to merge wip/exalm/dark into main
Reorganize and unify how styles are handled:
- Introduce a single style-variant setting, same as in gnome-text-editor
- Replace night-mode and follow-night-light with it
- Switch the style scheme along with it, like with follow-night-light
- Make dayhack and nightback toggle it instead of doing their own thing
Link to libhandy, so it can follow the system preference in GNOME 42.