Never had this happen. Probably won't ever happen to be honest, since my site's themes are so blatantly customised for my site that anyone stealing them wouldn't ever be able to get away with it.
I can honestly say that's never happened to me, though I don't usually purchase themes, I normally just make my own. If someone steals my theme then it's a copyright issue and I'll deal with it.
Part of the online world I suppose, best thing to do is continue the drive and working on your forum. Someone who is lazy enough to just steal an existing sites style probably won't last long on there site 😀
This hasn't happened to me, I don't buy custom themes, I just go with pre-made skins and customize them to my liking, of course within the license agreement.
I have seen this happen quite a lot though, Forum Hour.com has had this happen to their custom made mybb theme a while back.
It is a very serious issue that is actually annoying. I have never had it happen but i know forumhour has there custom theme stolen whether or not the thief has taken it down i do not know.