Really being able to see your site through a link would probably help if that is possible. However, you mention localhost so I take it you’ve not got it on a server. Diagnosing as a result is going to be tricky.
I take it you’ve confirmed it is theme not plugin (other plugins to BuddyPress) by testing with the default BuddyPress theme? Your comments seem to be saying that?
Outside of that it really is going to be a case of using something like Firebug or WebKit Inspector to try and pin down what is going on where. My guess is either an unclosed div or some scripting but I really can’t know without a link. Perhaps someone else has some better ideas I’ve not used that theme so probably if they have they can give better insight.