This guide is outdated and will not work the current GDM.
I’m surprised I didn’t figure this out a while ago because the answer has been staring at me in the face this whole time.
Have you ever gotten a very nice looking GDM theme only to be disappointed because the widgets and highlights were still themed in the Human theme and it clashed horribly? Like this?
Well there’s actually a very simple solution to this. If you go under System→Administration→Login Window, the General tab contains a check box that says “GtkRC file:”. This is where you can specify a gtkrc file for GDM to use so it will use a different GTK theme.
Most themes can be found in /usr/share/themes/ and the gtkrc file will be located in the gtk-2.0 directory under the theme name. In the example I used the Clearlooks gtkrc and afterwards my GDM looked like this:
Ah, much better. Happy theming!