how would you describe the theme
You can describe multiple themes from this story by examining characters and events. For example, the story explores themes dealing with mortality, friendship, and betrayal.
When discussing this story, it probably isn't appropriate to speak of the theme, because the story contains more than a single theme. "The Cask of Amontillado" explores themes that deal with death, mortality, revenge, betrayal, torture, and even alcohol.
In order to "describe" or discuss a theme, a reader should consider what events happen during the story and examine characters. Character actions can be examined in addition to character traits as actions often illustrate traits. Montresor is the one telling the story, so readers get good insight into his personality and mental state of being. Poe once again uses a fairly unreliable narrator, and we simply can't know whether or not Fortunato truly deserved his gruesome end. What we do know is that Montresor felt that he was wronged, and he planned out and executed a devious revenge. If you are going to explore a single theme, revenge and betrayal would be a top recommendation of mine. Explore what betrayal means. In order to betray someone, there had to be mutual trust. Fortunato fully trusted Montresor to do him no harm. He believed that Montresor had the amontillado, and even as the wall was completed, Fortunato still hoped that his friend wasn't actually betraying him. There can't be betrayal without trust and friendship, and the story thematically explores that relationship.