This guide is outdated and will not work the current GDM.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?
Ugh. 
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! 
Out of date, remove this crap please
This is not crap. Even if it were out of date, it would still be useful for people looking for old information. Plus, this is still useful for MDM. Try to be more respectful.