One common theme is the exploration of their relationship. Given their different personalities in the original work, fanfictions often focus on how they bridge the gap between them, like how Giyuu's stoicism and Shinobu's more cheerful yet calculating nature interact.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you can easily search for 'female giyuu fanfiction' using the search bar. Another option is FanFiction.net, which also has a good number of such stories.
One common theme is the exploration of their contrasting personalities. Shinobu is often seen as more light - hearted and talkative, while Giyuu is quiet and reserved. Fanfictions usually play with how these differences can either create friction or bring them closer together.
The themes usually include unspoken love. Since both characters are not very vocal about their feelings in the original story, fanfictions often explore the idea of them having deep, unspoken emotions for each other. Also, there's the theme of support. They support each other in battles against demons, and this aspect is often elaborated on in fanfictions, like how Shinobu's medical knowledge helps Giyuu when he's injured.