Heya!
There is no set theme beyond the general "Do something from the challenge list", and no absolutely does not need to be related to Halloween at all! The timing of jam was decided by vote and is just next to Halloween by accident!
As for double entry, as long as the entry hits our requirements and work isn't started before the start time, I have no issues with re-used entries. However if it's obvious it's being spammed to every jam I will remove it
Hope that helps!