Here is a way to change the looks of your login screen, it allows you to change the background, fonts and GTK theme using the Appearance Manager.
First open a Terminal window (Applications -> Accessories -> Terminal) then 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. To prevent the Appearance Manager from opening when you login, open a Terminal window (Applications -> Accessories -> Terminal) then copy+paste the following line:
sudo unlink /usr/share/gdm/autostart/LoginWindow/gnome-appearance-properties.desktop
If you want to re-install the Plymouth theme run the following commands from a Terminal window,
sudo aptitude install plymouth-theme-*
Now you can run the following commands to change login and plymouth screens
sudo update-alternatives –config default.plymouth
gksu -u gdm dbus-launch gnome-appearance-properties