Microsoft's Expression Studio Blend, Web, Design, etc. all have two themes -- Light and Dark.
Is it possible to obtain and use other themes?
There's a whole bunch of themes on the official Microsoft Expression Theme Gallery. Take your pick.
EDIT : My original answer is non-applicable - question asked was not themes for documents, but rather themes to change the look and feel of the app itself. Updated answer below.
Microsoft Expression uses a mix of native Windows UI elements, and also Windows Presentation Foundation elements. As such, it is really tough to edit even the base colors alone, so unfortunately, the answer is...
Nope, you can't haz more.