What are some good themes...?

I am currently hope elect in my local assembly for International Order of the rainbow for girls. Which means I have a year and a half to do a lot of planning. In order to begin all this planning, I must have a theme. Something to base all of the decorations off of. I need some suggestions. Help?! Keep in mind, it is not good to have a lot of black in your installation, and it should have a meaning to go along with it. It's been puzzling me for six months.