Oh, sorry. Unfortunately Ximian kinda goes their own way when it comes to the Gnome desktop. I'm not really sure if any of the themes out there will work with it without some tinkering.
If you're really into trying out Gnome my favorite place to go is here:
http://people.ecsc.co.uk/~matt/repository.html
and just apt in all the packages I need. Nyquist has more cool stuff available in one place than just about all other places in Linux-dom.
I really doubt you'll be able to install anything from there with Ximian installed though. The last time I looked into Ximian there were just too many conflicting packages to mix and match from other sources.
Sorry I couldn't be of more help.