Hello ldclan and welcome to the Ultimate Edition Community
There are two ways to change your Login Theme (Background etc.)
Way one:
Downgrade GDM and use TheeMahn's GDM maker to create your own Login Window.
Tutorial how to Downgrade:
viewtopic.php?f=65&t=4124TheeMahn's GDM maker:
http://www.youtube.com/watch?v=eqC7Ljege0MWay two:
Install GDM 2 Setup. You can use this tool to change the Background wallpaper.
Just Add this PPA and install gdm2setup:
ppa:gdm2setup/gdm2setup
Way three (not sure if it works): Use TheeMahn's XSplash maker to create your own GDM2 Login window.
TheeMahn's XSplash Maker:
http://www.youtube.com/watch?v=yat5-7zCy-EHope I could help.