Theme is not your character arc, nor is it the plot or what happens to your character. It's actually the essence that ties those two together. If someone asks you "what is your book about?" you don't respond with scene-by-scene detail, or the changes your character goes through.
You think of your character and what essential thing she or he comes to understand through the course of the book.
If you can't do that, you don't have a firm grasp on your story's theme.