The lack of a theme in your novel is lethal:
Before you start plotting your novel, you need to decide what the theme of your novel is going to be because that’s the safety line that will guide you throughout the whole conception of your book. Of course, some writers do not know their theme until they are half way through their novel, but, in the end, they still need a theme.
A theme is a recurrent idea in a novel. It is what you learn from the novel, the moral. It is what is left with you after you finished reading the novel.
Your novel will build around the theme. In other words, you will say through your words, “I believe this about life and I am going to prove to you I am right to think that way because that’s what happened to my characters and that’s what they found out.”
It is a thesis you need to prove.
The theme is different from the subject. The subject is what you base your novel on. It’s what you are going to talk about.
For example, the subject of The Wizard of Oz is a personal quest, but the theme is finding the resources that are already inside us. The story teaches children that nothing happens by magic, that their own actions control their outcome.
In each novel there is one main theme and different minor themes. In the Wizard of Oz, some of the minor themes are the lack of self-confidence, the belief in oneself as well as “there’s no place like home”, and the gap between appearance and reality (important people are frauds).
The author reminds the reader of the theme through the conversations his characters have between themselves, through events and through internal monologues.
Frequent themes in literature involve love, especially love is strong enough to overcome anything or we can resist any enemy, but not love.
Another theme is based on crime, especially crimes never get unpunished or the perfect crime does not exist.
These themes create huge emotional arcs that engage the reader. Without a powerful theme, your story is doomed because the theme is what makes a story worth reading. If your characters are not learning anything, why read their stories in the first place?
“Theme.” Literary Devices