Though it seems so basic to me now, I used to struggle with identifying the theme in the books I read.
A definition of theme is that it is the main message of the story or the central idea the writer is expressing. It is what the story really is about and why the story matters.
In other words, it is the deeper layer of meaning running through the story’s surface.
Theme is the glue that holds together a story. Otherwise, the story consists of this happening and then this and that and lacks that meaning.
As an early reader, I identified several “themes” in the text that, for me, added many layers through my interpretation of the unfolding events. But I couldn’t say what the main theme was, such as good versus evil or overcoming some difficulty to achieve success. I couldn’t narrow what I read into a few words, though I could summarize the plot from the beginning through the climax to the end.
The same goes with my writing. I just want to tell a story. I come up with character identities and a brief plot outline. It’s not until I start writing and thinking about my story that themes arise, usually more than one.
Theme is not just a simple idea fleshed into story; it is how the writer interprets the world. It is how the writer explains what people do as they interact with that world and with each other.
The writer doesn’t have to come up with some great revelation about human behavior but simply can offer some insight or comments. The writer can achieve this without being overbearing, preachy or heavy handed.
Readers, in turn, interpret the theme, or themes, differently by noticing different aspects of the story. The theme makes them think and ask questions about what they’re reading but also about their own lives.
It is the lesson or conclusion that can be drawn from the story that adds value to the greater world.
* See Zoey’s blog on the same topic at http://zoeyspaw.wordpress.com/2012/03/18/dog-themed-love/.
Shell's Writing Ink 
Once again a concise and though provoking walk through the mysteries of writing! You’ll likely make a writer of me yet. Thanks for sharing yr insight and wisdom.
Even as I write, it can be hard for me to summarize the themes in my own work! I know what they are, but telling them to someone is hard. Fortunately, they still come across in the story.
I struggled with “theme” too. After I read Larry Brook’s Story Engineering “theme” and the other 5 Core Competencies of storytelling because easy to understand. I’ve created a quick run-through the “theme” chapter here: http://rebeccaberto.wordpress.com/2012/01/02/the-best-advice-ive-learned-on-mastering-theme-in-storytelling/