I don't like themes.
We're talking about "game jams". A game jam is a time-limited event, where the participants create a video game. The goal is to spark creativity, learn more about making games, have fun, and bring the game development community together in a spirit of mutual support and friendship.
Most game jams have a theme, often in the form of a short phrase or a word (sound files, pictures, bits of video and other things are sometimes seen too...)