The new login theme may or may not be the problem, but we can find out by replacing the display manager (this isn't a big deal, and it's easy to undo):
Originally Posted by afbase
When prompted, choose to make kdm the default display manager. Then:
sudo apt-get install kdm
If this starts up the graphical environment successfully, you can then go into the Gnome tool for changing the login screen and revert it to its old settings. Then you can change back to gdm by running:
sudo /etc/init.d/kdm start
and selecting gdm as the default this time.
sudo dpkg-reconfigure gdm
If that doesn't work, please post any error messages you received after running the command to start kdm.