Idea for a Theme from the book below?

I have to come up with a theme for the book Whatever Happened to Janie. The book is about a girl who is kidnapped at age 3 and lives with her kidnappers parents who raise her. One day she sees a picture on a milk carton and recognizes it as herself. She has to decide if she wants to say anything to anybody, because she knows if she does she may have to leave the couple she has known to be her parents and live with people she doesn;t even know. She is now 15. In the end she is reunited with her birth parents but very unhappy. I believe in the end they take her back to live with the couple who raised her (who did not know their daughter kidnapped her - they thought it was their granddaughter). I have to come up with a Theme for this for a school project. Any ideas?