Take a look at this post to get a gist of how the built in themes work.

However, I don't think themes will help that much with what you're trying to do. Take a look at: http://www.alistapart.com/articles/alternate for switching between style sheets and remembering the selection in a cookie. You could have the regular style sheet and one named 'wide' and just allow the user to choose which one they want (or write some extra javascript to look at the window size).

Adrian