As long as you don't start working on the game or creating assets for the jam before the theme is revealed, you can brainstorm as much as you want to. 馃憤
It depends on the kind of person you are though.
I know my best jam games were made when I didn't have a plan in mind before the theme was announced, and my worst were made when I had a plan in mind before the start of the jam and tried to cram the theme into my original idea. 馃槄
