Family is a big theme. It's not a typical family situation, but it still explores the bonds between mother and daughter. There's also the theme of acceptance. Hermione has to accept this new part of her life and Bellatrix has to accept her role as a mother. In addition, power dynamics play a role. Bellatrix was a powerful Death Eater, and Hermione is a powerful witch in her own right, so how they interact with each toher in this new family dynamic in terms of power is an interesting aspect.
The themes often include secrecy and revelation. The fact that Hermione's mother being Bellatrix was a secret that is now revealed creates a lot of drama. Also, there's the theme of forgiveness. Can Hermione forgive Bellatrix for her past actions? And can Bellatrix forgive herself? These are all important aspects explored in such fanfictions.
One main theme is identity. Hermione has to re - evaluate who she is when she discovers Bellatrix is her mother. Another theme is redemption. Bellatrix might be seen as trying to atone for her past actions through her relationship with Hermione.