I was being a smart ass, but the larger point is if you're having a good time, the theme is secondary to being with people you enjoy, and if the theme interferes with you having a good time, you probably don't want to be at that party. There really isn't anything wrong with party themes, especially if the party host is the one that has a great time planning the party. I've always disliked the idea of having party guests show in costume because it requires work and, even though the host doesn't intend for it, it opens up those who can't find/afford a suitable costume or don't have the time to prepare one. I know it's being stickler (and it's unrealistic for a well meaning host to account for his or her party guests' individual neurosis) , but that's the kind of added complication that costume parties tend to have.