Hi,
Thanks for your questions! Answers below...
> Can we use pre-made assets?
Absolutely, although many people will create their own.
> Can we use pre-made code?
If you mean using game engines, libraries, templates for new games, etc. then that's fine. Taking a game you've been working on for months, or forking a game and changing some pixels is probably not in the spirit of the jam :)
> Can we sell the games (for the jam or post jam)?
Absolutely!
> Not so much a rule, but is there a twitter hashtag?
The official hashtag is #GitHubGameOff.
> Is the theme mandatory for entering? (I'm guessing not but it'd be nice to have some clarification)
Theme Interpretation will be one of the voting categories, but it doesn't have to be an obvious tie-in with the theme. I'm confident you'll like the one we're about to announce and won't be short of ideas :)
> Are games allowed if they contain 18+ content?
No restrictions on content, but if it's something I wouldn't show my kids or grandparents... then it may be less likely that it'll appear on our blog :) Please also see itch.io's note on adult content.
> What are the voting categories?
Similar to last year: Overall, Gameplay, Graphics, Audio, Innovation and Theme Interpretation.
Please let me know if you have any other questions. I'll consolidate all of these into a FAQ and make a sticky post shortly!
Cheers,
Lee