When you finish reading a book, ask yourself to sum up the book in a single word. For example, a single word for the book Little Red Riding Hood could be deception. Deception is the subject of the book.
Next, stretch that single word into a message: innocence can lead to deception. This is a theme for The Three Little Pigs.
Literature is an art, and the beauty of art is that it can be interpreted differently by everyone. The message of a fiction book is much like the message of a beautiful painting.
Sometimes an object such as a bridge or a large body of water will appear over and over in a book. The object could actually be a symbol or motif which represents the theme.
A bridge represents a change. If you notice your main character is going through a big change, you can be sure that the bridge is being used as a symbol.