The main theme is often about sin, secrecy, and the impact of hidden guilt. The black veil in the story serves as a powerful symbol of something being concealed, which could be related to the minister's own sense of sin or a more general concept of the hidden sins within the community.
It's about the idea of mystery and the unease it causes. The black veil makes people around the minister feel strange and uncomfortable. It might be about how we react to things that are not clearly understood and how our perception of others can be greatly influenced by something as simple as a piece of black cloth. So the main theme is related to mystery and perception.
Well, in 'the black veil story', the main theme could be seen as the isolation that sin or the appearance of sin can bring. The minister with the black veil is gradually cut off from his community. People are afraid of him or at least very curious and distant. It shows how a sign of possible sin can separate a person from society, and how society itself reacts to the idea of hidden wrongdoings.