On more than one occassion, I have observed writers run screaming in the opposite direction when the topic of theme is introduced. They vehemently object to writing stories with a theme claiming that they just want to entertain with their stories and not send messages to their readers. It's as if theme can only apply to "literary" fiction and classical literature.
So, what is theme? It is the main idea, moral, or message of a story. It doesn't have to be a moral or a message, it can just be the idea behind the story. Another way of looking at it is what is the writer's opinion of the story or what is the writer trying to say. Some examples of theme are love conquers all, life's a bitch and then you die, good triumphs over evil, evil triumphs over good, death, racism, salvation, etc.
I think the reason that some writers vehemently object to thinking about the theme of their story is because it seems a bit arrogant as if they're putting themselves on the same footing as Shakespeare, Hemingway, Mark Twain, or Edgar Allen Poe. When in fact, all stories have a theme. Even Dr. Seuss' stories have a theme.
"The Sneeches" is about racism and prejudice. "Yertle the Turtle" is about opposing a dictator. "How the Grinch Stole Christmas" is about the overcommercialization of Christmas. Dr. Seuss did not write those stories to teach children a lesson or a moral. He wrote those stories to help them learn to read.
One of my favorite Beginner Books when I was a kid was "The King, the Mice, and the Cheese" by Nancy and Eric Gurney. To me, the message of the story is to think of the consequences before you act. The King loves cheese and gets infested with mice. His advisors bring in cats to get rid of the mice, then dogs to get rid of the cats, then lions to get rid of the dogs, then elephants to get rid of the lions, then mice to get rid of the elephants, and the King was right back where he started and came up with a better solution to the problem. It's an entertaining story with a simple theme and no one in their right mind would put them in the same category as Hemingway or Poe.
Theme: it's not just for great literature.