Feel free to make some better looking themes.  A basic primer is that the system uses Smarty ( http://smarty.php.net ) for it's engine, and the existing themes should be a good example.

Really the only rule on building themes is fairly common sense, don't steal others work.  So in effect you can probably come up with anything you like.  I'm more nuts and bolts based, however I do appreciate a well themed work.  I regularly search the Internet for active HCL sites, and have loved some of the themes I've seen people come up with.