In 'creepypasta x reader fanfiction', a common theme is the exploration of the dark side. It's about the reader being exposed to and sometimes even attracted to the darkness that the creepypasta characters represent. There's also the theme of power dynamics. Usually, the creepypasta characters have some sort of power over the reader, which creates an interesting dynamic in the story. Additionally, themes of isolation can be present. The reader may feel isolated as they are the only 'normal' one in a world filled with these strange and often malevolent entities.
One common theme is danger. Since creepypastas are often horror - based, the reader is usually in a dangerous situation with the creepypasta character. Another theme is the unknown. The mysterious nature of the creepypasta characters adds an element of the unknown to the relationship with the reader.
Well, there are several common themes. Fear is a big one. The stories are designed to make the reader feel a sense of fear or unease as they interact with these often - terrifying creepypasta characters. Another theme is the allure of the forbidden. The idea of getting involved with these dark and dangerous entities has an allure to it. Also, there's often a theme of transformation. The reader may find themselves changing in some way due to their encounter with the creepypasta character, whether it's a change in their perception of the world or a more literal, physical or mental change.