As stated in the jam's description: "An optional theme will be announced once the jam starts."
So what is this optional theme? The jam already started, and I haven't noticed any changes to the jam's page yet.
I went to bed early and forgot to update the jam page, my apologies * facepalm *
The optional theme is TAROTS.
Well dang, now I want to write a tarot-based TTRPG instead of mucking about with Twine...
Ha ha, that's perfect. I already decided to write mine based on the Vampire Tarot I got when I was a teenager!