I randomly found a XenForo theme for IP.Board 3. While I actually like such look-alike themes, I wonder if it would be an interesting project to just port it to other board systems too, but I'm not entirely sure if the XenForo team would send me some C&D letter when I do it.
Don't take my word for it, but I don't think anything would happen. As long as it's not a copy/paste, and you've actually made the design yourself, I don't see a problem. The code will be different, and the code will be yours.