One common theme is friendship. Writers often explore how Lolbit and Mangle could become friends in the spooky world of FNAF. Another theme is adventure, like they go on an adventure together in the abandoned Freddy Fazbear's Pizza place.
Survival is another theme. Given the nature of the FNAF universe, they might have to work together to survive against the other animatronics or some unknown threat. There could be stories where they use their unique abilities to protect each other and make it through the night. Also, self - discovery can be a theme, where through their relationship, they learn more about themselves and their place in the world of FNAF.
Love and romance are also quite prevalent. Since it's a pairing, many fanfictions delve into the romantic relationship between Lolbit and Mangle. This could include things like how they first fell in love, or how they deal with the challenges of being in a relationship in such a strange and dangerous environment. Some fanfictions also focus on mystery, for example, there could be a mystery surrounding their connection or a hidden past that they share which gradually unfolds as the story progresses.