In 'Harry is a dark elf' fanfictions, a common theme is the clash of cultures. Harry comes from the wizarding world and now has to adapt to the ways of the dark elves. There's also the theme of acceptance. Will the other dark elves accept him? And will Harry accept himself as a dark elf? These are often explored in such fanfictions. And of course, there's usually some element of adventure as Harry embarks on new quests and challenges in his new form.
Themes like isolation and belonging are quite common. Harry might feel isolated as a dark elf at first, being different from the others. But then he might strive to find a sense of belonging. Also, the theme of transformation is often present. Not just physically, but also in terms of his mindset. As he becomes a dark elf, he has to change the way he thinks about magic, society, and his relationships. And many of these fanfictions also explore the theme of redemption, especially if the dark elves are seen in a more negative light in the original lore, and Harry has to change that perception.
One common theme is identity. Harry has to figure out who he is as a dark elf, different from his identity as a wizard. Another theme is power. Dark elves often have different powers than wizards, so exploring how Harry uses and masters these new powers is a big part of it.