Theme? Plot?
I think you'd be surprised at the number of people who show up, not just on the first of November but on the remaining days as well, sign up, and start writing
without a single thing planned.
No, no theme. No standard plot. No genre requirement. No requirement for cohesiveness. No requirement for logic. No requirement to have your main character alive at the end of it. No... requirements. Other than it be a novel that you started from scratch on or after November 1st.
If you wanted, you could write about a 75 year old drunken gay sailor turned artist on a killing spree in a fantasy setting in post-apocalyptic France with a young blind and paralyzed woman as his romantic partner. Preposterous is fine, too.