I haven’t really written anything yet because I’m struggling to come up with a theme for [the book]. Also, would you agree that all the stories have already been told before?
Yes, there’s really nothing new under the sun from a high-concept standpoint. Look at how many times the Pride & Prejudice tale has been reinterpreted: Bridget Jones’s Diary, Clueless, etc. Don’t beat your head into the padded wall over the singular goal of coming up with some idea that “has never been done before,” and then praise your own genius when you finally do, because at its base level, it probably has. Killer storytelling trumps originality every single time. Besides, the true originality is revealed in the details. How your particular characters, in all their quirks, respond to the situations. The metaphors you create and the words you use to flesh them out.
Theme should not be a target to write to. It’s like swinging at a pinata after you’ve been blinded and spun. Worse, it will probably reek of soapbox-prophet morality. Plot is what you’re actually struggling with (I hope). To me, theme is sort of a natural conclusion. It’s something you don’t really find until you’ve written a majority of the work already. In revision mode, a keen eye will notice some repeating elements, some common threads or undercurrents that run through the piece. Once identified, focus on just two or three of them and rework the associated passages to support or enhance those concepts.
If you agree there are a finite number of stories out there, then there are certainly only a handful of themes to go with them. Redemption, man vs. nature, revenge, love conquers all, etc. That’s why you shouldn’t start there; it’s far too vast to inspire. Begin with specifics, and broaden your scope as you go.
Unlike plot, theme is not something that should necessarily appear obvious upon a casual read, nor should it be required to enjoy it. It’s more of a feeling than a fact, and deepens the meaning once understood. Sort of like the right-brain’s interpretation of what the left-brain has told it.