I've been to many parties and honestly, the best ones haven't had a theme.
At these parties there are tons of snacks. We usually have a barbeque and there are about 15 people who are all really close. If you feel comfortable around the opposite sex invite them! Trust me, its much more fun. Have it later... till about 11:00 pm, then at night you can use glow sticks and run around your neigbourhood.
Hope I helped anyone who's stumped about party themes!