Sorry that the whole theme process isn't more clear at the moment! This is the first year that the jam is taking place so not everything has been set in stone yet.
By theme, I'm referring to a central idea that all games in the jam are encouraged to draw from. (Very similar to how Ludum Dare incorporates a theme into its jam).
I'm still mulling over how to select the theme. Ideally, it would be similar to Ludum Dare, where the community submits and votes on themes over a series of rounds.
Right now, I'm now aware of a good tool that allows us to do that while restricting voting to those participating in the jam.
What I may likely end up doing is starting a theme suggestions thread and building a top list from there. Participants could then vote on their favorite from the list and then we would have our theme.
I'm open to suggestions though!