One common theme is the adaptation of their traditional power dynamic into a modern context. For example, Sebastian's loyalty to Ciel might be shown in a corporate or academic setting. Another theme is the exploration of their relationship in a more open and accepting modern society. It could involve Ciel coming to terms with his feelings for Sebastian more easily than in the original Victorian - era - based story.
Common themes often include modernizing the supernatural elements. In ciel x sebastian modern fanfiction, we might see Sebastian's demonic powers being used in a modern technological world. There's also the theme of identity. Ciel may struggle with his identity as an 'Earl' in a world without formal titles, while Sebastian has to hide his true demonic nature in a more skeptical modern society. This creates a lot of interesting plot twists and character development.
A frequently seen theme is the clash between the old - world values they bring from their original story and the new, modern values. Ciel, with his sense of propriety and hierarchy from the Victorian era, has to adjust to a more egalitarian modern world. Sebastian, on the other hand, has to deal with modern laws and ethics while still fulfilling his contract with Ciel. This conflict often drives the plot forward and makes for some very engaging stories.