I don't really know what you mean by "Lock screen theme". The gnome-screensaver lock dialog uses the same GTK theme as the rest of your user session, which is already customizable in gnome-appearance-properties
I don't really know what you mean by "Lock screen theme". The gnome-screensaver lock dialog uses the same GTK theme as the rest of your user session, which is already customizable in gnome-appearanc e-properties