In my case, I use free Brivium themes for my many XenForo forums, though, I don't like all of them, only a few. Well, it's not to say I don't always dislike all of them, but they don't fit what I'm doing. Anyway, I use one premium dark XenForo theme, Yytrium for a math forum I have, but I think it would look better with a physics one, due to its outer space look.