To me, the main theme of this story is that people are not able to really know what is in the hearts of the other people that they meet. I think that this is why the story is entitled "Dusk" -- because people see other people as if it is dusk. The others' motives are only seen in dim light and we cannot be sure of what they are.
This ties in to the idea of deception. In this story, the young man deceives Gortsby. He is able to do this because Gortsby (like all people) cannot see into the hearts of other people. Gortsby is not suspicious enough of the young man and so he is fooled when he finds the bar of soap.
So a theme of the story is deception -- Gortsby can be deceived because he, like all people, lives in a kind of dusk when it comes to reading other people's motives.