I recently read Sidelines by Lois McMaster Bujold, which is a collection of essays, speeches, and travel writing. I found that it's geared more towards writers, but really, anybody who is a huge fan of hers would probably find this a good read.
There are quite a few gems in Sidelines, one of which includes this quote: "A true statement like 'plot is what happens, and theme is what the book's about' failed to create a distinction for me. It wasn't until I became a writer that it came clear. Plot is what a book's about. Theme is what a book's really about."
This was certainly true for me. Back in high school English, we'd read something, and then the teacher would discuss the theme as if it were some hidden message put there by the author, and only the clever people were capable of figuring it out. I felt like there was a code, or maybe a key, that would allow me to figure out the theme (and thus pass the darn test). But theme is both more complicated, and easier, than that.
Many authors don't even know their story's theme, or themes, until after it's written. I might have an inkling of theme when I write, but often I don't have it figured out until afterwards, and then I go through and make sure the story supports it in character action, symbolism, dialogue, etc. Then I hand the story to another person who has a different life experience than me. They bring that with them as they read, and they might find a completely different meaning in the story, and that's okay. Phew. It took me a while to figure that out.
So what is a story really about? It could be about repressive Victorian ideas, or that love conquers all, or a political diatribe, or a comment on how science will make or break humanity, or the role of religion in a person's life, and so forth. There are many meanings that a story can hold. One of the exciting things about having other people read my work is what they bring to it. Sometimes a reader comments upon something in a whole new way, and I see my own story in an entirely new angle, and that's just pretty darn cool.