I think the theme should be Pirates vs. Ninjas.
Everyone attending has to dress as /either/ a PIRATE /or/ a NINJA. There is to be NO conversing about who's coming dresssed as what. Then on the day whichever lot has more people (that is if there are more PIRATES than NINJAS or more NINJAS than PIRATES) gets to make the other lot (PIRATES or NINJAS) buy them beer.
Failing that I suggest a Hello Kitty theme. I'll be the humiliated one in the corner.