Ubuntu's GDM still does not have a settings window where you can change looks to your liking. However, there are some tricks to still be able to do just that.

First, open a terminal and type the following command:

sudo cp /usr/share/applications/gnome-appearance-properties.desktop /usr/share/gdm/autostart/LoginWindow

You may also want to do the same with display-properties.desktop for setting the resolution of your screen and gconf-editor.desktop to change apps > metacity > general > button_layout to something like "/apps/metacity/general/button_layout".

Then logout, and you'll see an Appearance window pop up (and the other windows if you have copied those .desktop files too). Change it to how you prefer it, then close it and login as usual.

Now, to prevent those windows from appearing every time you get to the login screen, remove the .desktop files again:

sudo unlink /usr/share/gdm/autostart/LoginWindow/gnome-appearance-properties.desktop

And repeat for the other one or two, if you used them:

sudo unlink /usr/share/gdm/autostart/LoginWindow/display-properties.desktop
sudo unlink /usr/share/gdm/autostart/LoginWindow/gconf-editor.desktop

Done!

[Based on this how-to]