GTK4 removes the generic GtkWidget API for accessing an inserted
action group, so we need an alternative for tracking the currently
selected theme.
Using the underlying GSettings object looks like the easiest option,
so do that.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/148>
While we don't endorse or support 3rd party theming, the extension
exists and is actively used. However right now the most convenient
way of setting it up is by installing Tweak Tool; give users an
alternative by providing a simple settings dialog ourselves.
https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/117