Hacking Exhibit Themes in 1.2

We're looking to build a central exhibit site for smaller projects here at GMU Special Collections & Archives. Our goal would be to have a customized overall site theme, with individual customized exhibit themes. We haven't yet worked with 1.2, and I am wondering if you can hack the individual exhibit themes, or are you limited to the standard choices? We'd want to have multiple hacked themes running at the same time.