Think of theme as the "meta-message," the one big statement about the world your work of fiction will convey. It will do this whether you’re conscious of it or not.
A novel will have only one meta-message, though it may offer several sub-messages. For example, the theme of Dostoevsky’s The Brothers Karamazov is "Kindness and justice are the highest values of human existence." But the novel propounds numerous messages, such as the futility of pure intellect and the burden of free will.
But you don’t want characters who merely represent a thesis. They must be real people with passionate hearts.
Your job is to find out where that passion is. In your prewriting, allow time for your main characters to explain, in their own voices, a philosophy of life.