The basic procedure was to have the login screen open, login through a tty and open the gnome-control-center with sudo as the user gdm.
There you could easily change the look and feel.
Now that I upgraded to 11.04, I ended up with the following error:
$ sudo -u gdm gnome-control-center
(gnome-control-center:4136): Unique-DBus-WARNING : Unable to open a connection to the session bus: Failed to connect to socket /tmp/dbus-rTZcGEoY9e: Verbindungsaufbau abgelehnt
(gnome-control-center:4136): Unique-DBus-WARNING : Unable to connect to the running instance, aborting.
But well I dig a little bit and came up with an alternative procedure, with the same basic idea.
2. Open a terminal
3. Give gdm a shell access:
$ sudo gedit /etc/passwd
Find the line saying something like
gdm:x:106:114:Gnome Display Manager:/var/lib/gdm:/bin/false
Change it to something like
gdm:x:106:114:Gnome Display Manager:/var/lib/gdm:/bin/bash
4. Grant localhost access to the X-Server:
$ xhost +LOCAL:
5. "Login" as gdm
$ sudo su gdm
6. Export Display-Variable (you may want to check it in an other term with echo $DISPLAY)
$ export DISPLAY=:0.0
7. Open Settings
And now: Happy customizing, because now you can change everything for the gdm-user and therefore the login screen
Don't forget to change back the shell for gdm to false again
Edit: Found an other method to customize GDM at http://ubuntuforums.org/showthread.php?t=1747298&highlight=gdm+customize :
1. Let the the settings pop up on login-screen
sudo cp /usr/share/applications/gnome-appearance-properties.desktop /usr/share/gdm/autostart/LoginWindow/
2. Logout & and have fun with the appearance-settings
3. Login again and bring everything back to normal
sudo rm /usr/share/gdm/autostart/LoginWindow/gnome-appearance-properties.desktop
Advantages first method: You can easily install new themes
Advantages second method: You see what you get