What is the difference between "Something in Common" and "By Theme(s)". I see these appearing in various categories, like Entertainment, and I can't quite see the difference.
The latter is one of Entertainment's main subcategories. In it you'll find quizzes with a mixture of media forms all linked by a common theme.
I'm not sure where you're looking for the former. TV & Movies Mixture? If so, the quizzes in that subcategory *ONLY* involve TV and Movies, not Books, Video Games, Music, etc.
The difference is that you need to dig a bit deeper into the expectations of the subcategories (the guidelines) to better place the quiz. If you're having trouble doing so, the editors will be able to help.