I would say, "Writing, photography and art embrace the themes of..." I would see them as different singular entities, discrete element of a cohesive list. Of course, what those three things can possibly have in common and collectively embrace, I could not say. They often seem at odds, one with the other. I would dispense with "The" at the beginning of this sentence since you are talking about these three art forms generally and not something specific. Plus, there are some who would quibble with the idea that photography is not art. Just something to keep in mind. (My last sentence is not a complete sentence, is EXTREMELY informal, and not to seen as proper English). In American English parlance, "Pardon my French."