Actually on day one every single theme that we launched, I designed.
We gradually added the best themes that we liked the most, some of them were Tumblr themes, and others were created by Posterous users, e.g. http://corywatilo.com
I wouldn't even call them "Tumblr themes." These were just blog themes, owned by a third party, who licensed them to us for use on Posterous. They just happened to be on Tumblr first, but don't have any other affiliation to that service.
We gradually added the best themes that we liked the most, some of them were Tumblr themes, and others were created by Posterous users, e.g. http://corywatilo.com