A theme is a central idea to story. Stories can have more than one theme.
The theme of a story becomes clear as characters in the story respond to challenges. By the resolution (end) of the story, you get to see how the main character overcame challenges and failed at challenges.
How did a character choose to handle the challenges he or she faced? The answer to that will give you clues to what the theme could be.
This Week's Prompt:
Determine a theme of a story (from a book you have finished recently). Explain how a character in the story responded to challenges and how his/her choices teach you the theme.