Theme isn't particularly well understood. In fiction, it is not a lesson for readers to learn. That's a sermon or the way teaching material is organized. Nor is it a topic. I've often thought of theme as a world view the writer is dealing with.
The Difference Between Subject and Theme by Steven Pressfield does a very good job of describing the difference between, ah, well, subject and theme, just as the title says. Pressfield does a whole series on theme at his blog.
Also check out Grace Joliffe's post Theme at Practical Creative Writing. Theme, she says, is about meaning.
Many times writers will say they aren't aware of theme until after they've completed a work. These two articles help explain why that's not a good idea.