Selecting a theme depending on author

I want to implement a multi-user blog without resorting to MU or lyceum. My blog is already up and running (http://www.reign-of-the-rathalos.com) and I have three contributors so far.

My goal is to use subdomains to forward to my blog using the author’s name and optionally a theme only to be applied when that particular author is filtered. Author filtering seems trivial, however the theme setting is not.

I found the theme related wp filter but I’m not sure how and which to apply. Can anyone point me in the right directions?
Thanks