This is not as hard and confusing as it may appear to be. The theme is the essence of the story, the axis around which the narration turns. The theme is not something concrete, but rather an abstract concept, universal or something that everyone can understand: suffering, heroism, violence, cruelty, etc… As we can see, these are all words, aside from individually transmitting positive or negative values, that are not attached to a predetermined time or space. The theme of our story is what remains from it when we reduce it to a single word.
If you’d like to write a romantic story, it doesn’t matter if the lovers are Romeo and Juliet, two aliens made from burning lava, or a toaster and a microwave, the theme of the three stories will be “love”. To clarify this concept, let’s see what is the theme of these known stories:
Peter Pan, by JM Barrie: Childhood.
The count of Montecristo, by Alexandre Dumas: Revenge.
The picture of Dorian Gray, by Oscar Wilde: Narcissism.
Madame Bovary, by Gustave Flaubert: Adultery.
Crime and Punishment, by Fyodor Dostoyevsky: Guilt.
In conclusion, the definition of a theme is essential to the planning of a novel. Thus, this is what the novel is about and you’ll be working on this concept throughout the project.