The theme will be announced one week early in the Shacknews Chatty.
If you check out the Shacknews Chatty at 6pm Pacific on September 27, you'll get to see the theme early.
https://www.shacknews.com/chatty
(I recommend having an account and being signed in just in case the announcement post gets tagged "Stupid" or "Political," as those themes are suppressed from anonymous users.)
The theme will be announced in the normal channels on October 4 at 6pm Pacific.