One of the principle themes is that of blindness. This is shown through the character of Robert, who is literally blind. However, although Robert is the only character who is literally blind, at the same time, it is clear that the narrator is "blind" metaphorically in a number of ways. This is shown through his selfishness and also his inability to see how his actions are impacting his relationship with his wife, who is increasingly annoyed by his insensitivity and rudeness. However, it is when Robert asks the narrator to draw a cathedral for him, the narrator discovers that being blind and having to rely on your other senses can actually be a very positive and amazing experience. Note the way that when Robert urges the narrator to open his eyes and have a look at the drawing he made, the narrator doesn't:

But I had my eyes closed. I thought I'd keep them that way for a little longer. I thought it was something I ought to do.

He finds this experience incredible, as although he knew he was still inside his house, he "didn't feel like [he] was inside anything." The narrator moves from seeing blindness as a disability and making fun of Robert to actually being able to understand how blindness is something that can deepen one's perspective on life. In the same way, although the ending does not reference this, the reader gets the feeling that this visit from Robert will have changed the narrator's relationship with his wife for the better. The notion of blindness is therefore one of the key themes of the short story.