Changing your login screen works the same as Ubuntu 10.10 “Maverick Meerkat”.
To enable the Appearance manager at login open a Terminal window (press Alt+F2 and type: gnome-terminal) and copy+paste the following line:
sudo cp /usr/share/applications/gnome-appearance-properties.desktop /usr/share/gdm/autostart/LoginWindow
Now close the Terminal window and logout, when logged out the Appearance window pops up. Here you can make the changes you want and when your done you can login as usual.
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To prevent the Appearance Manager from opening when you login, open a Terminal window (press Alt+F2 and type: gnome-terminal) then copy+paste the following line:
sudo unlink /usr/share/gdm/autostart/LoginWindow/gnome-appearance-properties.desktop