What do I do if someone steals my designs ?

OK I recently did a new design for my tumblr site. Yesterday I found out that this guy just basically stole my design. His site looks exactly the same as mine except he uses a different header image. However he does admit on his site that the design was done me by having a link at the bottom that says “The design was done by this guy”. The ironic part of the this whole situation is that this guy seems to be a graphic designer and his tumble-log is about “effortless design”.