I am using Debian Squeeze Testing and I have gdm3 installed. I wanted to change the default gdm theme and here is how I did it.
I opened the file /usr/share/gdm/greeter-config/20_debian in a text editor as root and changed the line containing “/desktop/gnome/background/picture_filename” and gave the full path to the new location of wallpaper. I also changed the line containing “/desktop/gnome/interface/gtk_theme” and wrote the name of theme instead of the default (in my case it was Clearlooks and I changed it to an installed theme Equinox). Then I saved the file and logged out to see the changes.
While changing the theme, make sure that the theme is installed as available to all users (i.e. in /usr/share/themes).