Homecoming Dance themes?

So I was wondering, what are the themes for? This yeah, we have Narnia as our Homecoming theme. Does that mean like everyone dress as a character from that, or is it just how they're going to decorate it? There are four different themes for the four grades. I'm confused, if it's to have everyone dress up as a character, then dozens of Lucys will show up, and Edmunds, and so on...Oh, btw, I go to high school in the US on the west coast, if that contributes to anything. Thanks in advance!