A frequent theme is the re - imagining of the magical world. With Voldemort in a different role as an adoptive father, the power dynamics in the wizarding world change. Also, the theme of identity is prominent. Fem Harry has to figure out who she is in this new situation where she is associated with Voldemort. And there's often the theme of moral ambiguity as she may be exposed to dark magic yet still hold on to her own sense of right and wrong.
The theme of forbidden love also often appears. Given their status as enemies in the original story, the idea of them having some sort of romantic or deep emotional connection in fanfiction is a popular one. It adds an element of drama and tension to the story.
The theme of trust is often present. Given that Harry and Voldemort are usually enemies, when it's a female Voldemort, the story might focus on how they learn to trust each other or not. And the theme of changing fate. They might try to change the fate that would usually lead to their enmity, for example by working together to prevent a greater evil.
Well, the theme of identity is quite common. Fem Harry may struggle with her identity in a world that has different expectations for females. There's also the theme of loyalty. Sirius is extremely loyal to fem Harry, and vice versa. And often, these fanfictions explore the theme of love in different forms, whether it's the love of a father - figure (Sirius) for his adopted daughter (fem Harry) or the love that fem Harry has for her new family and friends.
Love and hate are also common. The complex relationship between fem Harry and Voldemort can be filled with both love and hate. It could be a romantic kind of love or a more platonic love - hate relationship. Identity is another theme. Fem Harry may have a different identity in the wizarding world compared to male Harry, and this affects how she deals with Voldemort.
The theme of redemption can also be found. There are stories where Voldemort might be on the path to some form of redemption through his interactions with fem Harry. Also, the idea of forbidden love or a strange attraction sometimes pops up. It's a very different take on the original relationship between Harry and Voldemort. It challenges the traditional view of good and evil. And in some fanfictions, there's the theme of sacrifice. Fem Harry might have to sacrifice something dear to her in order to defeat Voldemort or change his ways.
One common theme is family. Since Sirius is adopting Fem Harry, it's about creating a new family unit. Another theme is protection, as Sirius has to protect Fem Harry from various threats.
One common theme is redemption. Since Tom Riddle later becomes Voldemort, the idea of fem Harry being able to change his path and redeem him is often explored. It gives a sense of hope that even the darkest of characters can be saved.
One common theme could be family. Since it's about adoption, the idea of creating a new family unit or exploring the concept of family in a different way might be prominent. For example, how the new family dynamic affects Harry's character growth.
One popular 'Voldemort adopts Fem Harry' fanfiction is 'The Dark Lord's Daughter'. It has an interesting take on how Voldemort's character would change with a daughter figure. Another one could be 'A Different Path for Fem Harry' which explores the magical world from a new perspective with this unique relationship.
One common theme could be power dynamics. With Voldemort as Harry's master, there would be a complex play of power, dominance, and submission. Another theme might be redemption. Maybe the story could explore if Harry can somehow redeem Voldemort or if Voldemort can find a different path despite being in a position of power over Harry. Also, identity confusion could be a theme, as Harry has to deal with being under the control of his archenemy in a new and unexpected form.