So this can be on any genre, right?
I'm making a game, where player should survive some counts of days on cold planet. It's ok?
The point of the game jam is to make a new game within the time constraints of the jam.
Yes, that wouldn't technically be allowed. Submissions to the jam should only be started after the jam starts, for the reasons Carnny already stated.
Cookies don't inherently relate to the theme of "chill", but if you can find a way to tie to the theme then yes, technically. Though having watermarks over images may impair your game visually.
Cookies in milk <3..... cold milk.
Chill can be used in any sense of the word. So it could be literally cold, laid-back, or anything else you can think of that fits.
If you can work it into theme then yeah, horror is totally allowed!