Question

What theme does Shusterman explore in the novel Challenger Deep?

Answer 1

I have not read the novel, but a synopsis I found online seems to suggest that the story explores themes of mental illness (https://www.storyman.com/books/challenger-deep/).
You, having read the book, may verify whether that is true, and maybe you'll pick up on some themes for yourself.

As a reminder, a "theme" in a work of art is an idea or element that continuously appears throughout the work; this is also true for novels. If you notice any recurring ideas or elements while you read, that might be an example of a "theme".
For example, a theme of the cartoon Tom and Jerry is the competition between the titular characters and the ways in which that competition encourages dependency on one another.

I hope that can help you understand so you can answer your question. And I hope Challenger Deep was an enjoyable read (like I said, never read it myself) :)