Change Gnome login background/theme

1.When logged out, hold down CTRL+Alt+F1
2.Type export DISPLAY=:0.0 and press ENTER
3.Type Sudo -u gdm gnome-control-center and press ENTER
4.Hold down CTRL+Alt+F7
5.Gnome Control Center will appear. Press ''Apperance'' and choose a picture.
6. Close the window and log in