I installed adapta-gtk-theme and adapta-kde. In System Settings > Application Style > GNOME Application Style (GTK), if I set Select a GTK3 Theme to Adapta, and apply the change, the System Settings crashes. When I open it again, it reverts to the Breeze theme.
On the command line, I get the following error:
(systemsettings5:1560): GLib-GIO-ERROR **: 15:26:09.335: Settings schema 'org.gnome.desktop.interface' is not installed
It works for the GTK2 theme.