The grand majority of The Tell-Tale Heart is set in darkness: the narrator hides in darkness every night as he watches the old man, and the old man sleeps in the dark. This limits the characterization of both the narrator and the old man because it is hard to see in the dark, so the most striking character description we get is the bright blue eye of the old man, which seems to nearly glow in the dark, so that is what the narrator is focused on intently. The theme is also mirrored in the setting because both are dark. On the surface, this story is about a murder, which is a dark topic; in a deeper analysis of this work, it can reflect the darkness of man. But that is not necessarily the only theme of this story. The dark mood is also reflected in the dark setting. What I have said is only one way of looking at this story - in order for you to form a more comprehensive argument that you can truly get behind, I would suggest reading through the story and seeing what you think, how it makes you feel.