1. Logout of your current session and return to the GDM
2. Switch to the tty command line prompt using Ctrl-Alt-F1
3. Login using your normal login/password
4. at the command line prompt type: export DISPLAY=:0.0
5. then type: sudo -u gdm gnome-control-center
6. Switch back to the gdm screen using ALT-F7 ou ALT-F8
7. The gnome-control-center should be loaded. Use it to configure your GDM.
8. Click on the Appearances icon, in appearances you can change your GDM's font, theme and background image.
9. Close the gnome-control-center and login normally.