A theme is often an idea that is mentioned over and over again in the book (at different parts in the book). Take Catcher in the Rye for example. The name itself is a clue to the theme of the book. Throughout the novel, the song that mentions the children, which he misinterprets, correlates directly to the actions he takes with his sister and those he perceives as younger than him. The theme would be "The protection of the innocence of children takes precedence over everything. He often refers to himself as different than those with the child-like innocence because he is in his view. The theme is very apparent, as you read the novel, as it is mentioned over and over; you just need to pick it up.