Your novel will have themes, either through your own conscious choice or more organically. The following are the two most helpful books I've found on understanding and choosing theme: (1) K.M. Weiland's Writing Your Story's Theme and (2) John Truby's The Anatomy of Story I've written detailed notes on what I've learned from these books and from the writing workshops I've taken over the years, which you can download as a pdf below.