The theme has already been announced, and the game jam is still 2 hours away. So I can start developing now? Or would it be unfair?

It would technically be unfair yea, but this jam is not ranked and mostly for fun, so I don't think it's that big of a deal if you want to start a little early.
I'd wait to see what the host says tho.

This is a fun gamejam. No problem.