Hi! If I were you, I would refer to notes given to you by your teacher, as I am not entirely sure what level this is or what your teacher is looking for, or how your teacher may define a theme.
Personally, I would choose:
A theme is a message the text conveys.
A theme is supported by details in the text.
A theme is seen over the course of a text.
Again, be sure to double-check with your notes and/or teacher to see how they define it, but in my opinion, I would choose those three!
Hope this helps!
-K
