Your story’s theme is not the same as its plot.
Theme is the deeper meaning, the underlying message, behind your story, and much more Universal.
In three of Jane Austen’s novels, the themes are right there in the titles: Pride and Prejudice, Persuasion, and Sense and Sensibility.
When attempting to define your own themes, think in terms of a phrase, or sentence. Maybe two sentences, but rarely more.
Theme isn’t reserved for Literary Fiction, or any particular genre.
Ideally, theme should be identifiable in any story we create.
Sometimes, our own themes will be obvious to us, but at other times, not at all. We tend to bury the deeper meaning, and may find it difficult to pinpoint.
Whilst it’s possible to tell a great story without specifically considering the theme, it does help to know the intentions, reason, and purpose, behind what we’re doing, as writers.
What compels us to complete a particular novel? What, ultimately, are we hoping to achieve through our works in progress?
Theme is essential, because it’s what makes your work special. Unique.
A plot is a series of connected events.
Characters are of central importance, because they are the people aspect, and the particular people experiencing the events in question.
But, to tie the whole together, there’s going to be a why.
Why should readers be following these specific people, as they go through these specific life events?
What can the reader expect to gain from reading your work?
What will they learn, on an emotional, or even spiritual, level, that can be applied beyond the limited scope of the story world?
These are some brief thoughts, on the subject of theme in stories. Hopefully, this will help you, as you begin to consider and explore your own themes.