We often think of the theme of a piece of fiction as a statement of the story idea. That's not necessarily what we're looking at here. Developing a theme or themes when writing fiction is not the same as developing an elevator speech to succinctly tell someone about your novel. Rather, a theme is what underlies your story idea. For example, your idea may be to tell the story of the relationship of a father and his son. The theme that runs through the story is not that the father and son have a relationship. It's the guilt or forgiveness or whatever emotion or underlying human quality drives that relationship. These aspects of the relationship -- these themes -- illuminate the meaning of it and add depth.
Some authors use opposing themes to enhance their writing. We'll see a good example of this if we examine the themes running through the movie It Could Happen to You with Nicholas Cage, Bridget Fonda, and Rosie Perez. The story is about a husband and wife who win the lottery. His (Nicholas Cage's) theme is generosity. He gives a waitress (Bridget Fonda) a 2 million dollar tip. His wife's (Rosie Perez's) theme is greed. She sues to get all of the money for herself. These themes, together, drive the story. Individually, each theme dictates the actions of the characters.
As a writer, the themes you develop will act much the same as your car's GPS system. They'll keep you on track and show you just where to go with your story. In addition to keeping you on track, the themes will help you determine what is important to your story and what can be left out.
So, before you write your novel, think about what themes you want to express through your story idea. For example, don't write a story about insecurity. Instead, write a story about a character who is insecure. If you create characters who care and who express aspects of the human condition, themes will evolve naturally. Watch for them and capitalize on them.
Once you develop a theme or themes, every aspect of your story will be formed by them. You characters, their actions and reactions, and your plot will be moved forward by your themes. Consequently, your readers will be moved by your story because it will show the depth of the human condition.