GDM Tweaker is a very simple application that allows tweaking the GDM login screen look: change the GDM background image, hide the username list, enable Compiz and change the GDM login screen icon or GTK theme.
Sure, you can change some of these options using Ubuntu Tweak or you could run a command that pops up the Appearance Preferences and you can select the GDM login screen theme but that's a bit complicated. And since GDM2Setup doesn't work since Ubuntu 10.04, GDM Tweaker is among the very few applications that allows you to tweak the GDM easily.
GDM Tweaker won't allow you to use the fancy old GDM themes - that doesn't work with the new GDM. Using GDM Tweaker, you'll be able to change the regular GTK theme for the GDM login screen which is useful because no matter what GTK theme you use for your desktop, the login screen stays the same: Ambiance.
Install GDM Tweaker in Ubuntu
To install GDM Tweaker in Ubuntu 11.04, 10.10 or 10.04, run the commands below in a terminal:
sudo add-apt-repository ppa:madnessmike/madnessmike sudo apt-get update sudo apt-get install gdm-tweaker
Note: to be able to change the GDM background / theme you must place the images (it seems the images must be .png or .jpg) into /usr/share/backgrounds/ and the themes into /usr/share/themes/ for GTK themes and /usr/share/icons/ for the icons. TO be able to copy the themes / backgrounds to these folders, open Nautilus as root using "gksu nautilus".