"The theme should be the consistent driving force of the plot."
-Writing for Comics & Graphic Novels with Peter David, pg. 64.
When I started writing for comics, I thought had everything in order with the usual essentials. Plot. Setting. Characters. Story structures. Everything. But in recent years, I came across something else that plays a central part of storytelling and it's theme. There have been many discussions about how it works in fiction and the role it plays. Most of the time it's the "moral of the story" but it's not always the case.
When you look at Buffy: The Vampire Slayer, you think Sarah Michelle Gellar fighting the scum of Sunnydale's nightlife. That's just only the surface of the series. What the series is truly "about" is the highs and lows of high school from teen issues, desperation for acceptance, and what happens when you sleep with that "nice" boy. Basically it appealed to anyone who thought high school was hell.
As I think about my own stories, I wonder what themes did I put in without even knowing it. The first thing in my graphic novel I found was redemption. A member of my book's supporting cast was inspired by a relative who was sent to prison for a few years, and he decided to keep his nose clean when he got out. Seeking redemption is a relative thing for all of us when wrongdoings of the past are hanging over us. What makes my secondary character "real" in that regard is how he's looking to build on a second chance despite the fact what he's done will never be erased.
The theme of the story should be as solid as the story's plot. If it's not as strong as your plot, then your story can't sustain itself. What you should do is think about what you're going to "say" in your story while planning the story in your mind. For me, nothing's important as what you're "saying" in your fiction.