I believe if you're writing a book, it has to be about something YOU'RE passionate about. Something you have knowledge about, something you feel comfortable writing. If you're not passionate about the subject, you'll get to a point where you'll have difficulties to develop that theme, and you won't be able to finish your writing.
So, I truly believe a theme is something the author needs to come up with themselves. It can be anything, as silly as you may think that sound. What makes a good book isn't the theme, it's the way the story is told, how the author shows the story, how the author is passionate about it and how they develop the charatcers. Writing is a very personal process.
I know this isn't what you want, but I thought I'd give my input.