To me it's like an open-book exam. You're not allowed to look at the answers beforehand but you can bring notes with you into it.
Basically, don't create anything you intend to put in your game. Setting up reference material and doing practice runs would be helpful. I'm not running the jam so I'm not 100% it's allowed, but that's my view on it.
But always be aware that the theme could potentially throw all your ideas away, and that the addition of the theme could give you better ideas than what you came up with before knowing it.