Theme song? Well, it'd definitely be instrumental and orchestral. Like something from Two Steps From Hell. Maybe 'Breathe' Or 'Blackheart' 'For The Win' You know. Something epic! Edit: Now that I realize you said 'theme' song, no, I don't know what would be a good theme song for my series. However, I can see an instrumental (like the ones above) being played during each episodes' intro credits.
Though its not an actual song, I think its the music that captures the tone of what I'm trying to write.
A good theme song for my fantasy story is from my favorite VN(Visual Novel) called Rewrite. My fantasy story is about discovery inside a somber world where bits of sweetness hide inside a hollow bitterness. A good fitting theme song for my sci-fi called Ex-Human, comes from the Ar Tonelico game series. It's an appropriately dramatic hymm that describes my story a little too well.(In an alien language even)
I'm writing a novel in the genre of mystery, crime, suspense and psychological thriller. End of major chapter that reveals bitter truths a little too late:
I know it's already associated with a Bond movie, but I feel it's fitting for my novel. Die Another Day by Madonna.