The main theme of this amazing short story by Poe is Death. He comes in quietly, like a thief, disguised appropriately for this masquerade ball. One by one, the guests drop dead and the rest retreat to other rooms. The purpose of the party was to keep all the wealthy friends far away from the plague that is sweeping through the city outside. Prospero, like his wealthy friends, thinks that because he is wealthy and "above" the poor who are dying ouside his walls, that he is above dying. The truth is, no one can escape Death, no matter how rich, how beautiful, how entitled one is.
Another theme is time. Throughout the story, the clock keeps ticking loudly, and Prospero and other guests keep noticing the time. It is almost as though they expect an expiration date on the contagion. There is, however, an expiration date on the living. Everyone dies eventually. It is interesting to note, that the different rooms in Prospero's house correspond with the different stages in one's life. Take a look at the colors of each room...all the way to the last one, which is black, signifying Death itself.