Hmm, will look into it

Ok, so I can't reproduce the error. If I remove the .config/gtk-4.0 folder, it just gets recreated by my app and no error occurs on my arch machine with gnome.

If you selected the adwaita theme, yes, there will be no css written

yes it works on all desktop environments.

gtk4 themes are similar to the gtk3 ones - it's still css, maybe with some addidtions