Navigation in the header, ad up top, logo on the left, one smaller column with search, 300x250 ad, top users, and recent posts in the forums.
Main content area should show the most recent submissions, and then top/popular submissions (just like opsds). I'd ask that you:
1) make sure the thumbnail sizes change since the large thumbnail on the detail screen will be 400x240, you'll want to make sure the thumbnail stays proportionate.
2) Just create some top bar that allows the users to filter what they see per page and have pagination on the bottom, just like the browse page found here: http://alturl.com/kwx2
Theme detail page will follow the ithemer detail page moreso than the opsds detail page in that I'd like a frame of the actual sidekick device with the theme thumbnail/preview on top of it, like here: http://alturl.com/jghw
It should still have the stats (uploaded, downloaded, rating etc), commenting, and if space permits, a similar themes panel under it just like here: http://alturl.com/hpzk
Will need a footer for the typical stuff too like home, privacy, contact etc. Site will also have a forum (as mentioned for the smaller column) so keep that in mind for navigation.
As for style, we're open, but really like the colors and edgy design of Danger's site: http://alturl.com/r9om