They only have a few options and when you compare to other blog services it seems very lacking. I'm hoping there's some sort of repository out there I haven't bumped into yet.
Posterous blogged about this here: http://blog.posterous.com/posterous-theming-its-here-its-live-and-its-t
In this example they show how to port themes from other blogs http://posterous.com/theming/porting
You can check out this post made by Speckyboy, The Best Free to use Posterous Themes.