The moral (or theme) of "The Necklace" by Guy de Maupassant depends on your view of the short story. For instance, we early on learn that Mathilde longs to be a part of the upper class. All she wants is to go to the party and have a great time. When her dream comes true, reality leaves her crushed when she realizes that she lost Jeanne's necklace. Instead of beginning a life of grandeur in the upper class, Mathilde plummets all the way to the lower class as she has to work harder to make up for the loss of the necklace. This theme would be an example of man versus society - or the conflict between classes within a society.
Another potential theme maybe the conflict between generosity and greed. Who is generous in the story? Mathilde so longs to be a part of the upper class that she covets what she does not have. Even though she has more than some (one servent) she wants more and more. This is a sign of greed. Contrast that with her husband's selfless act of giving up what he wants (the hunting trip) in order to make sure that Mathilde gets her dream. Even Jeanne is generous in allowing her friend to use the necklace for her special night. By the end of the story, we can see the moral of the story for this view is to be careful what you wish for because the grass always looks greener on the other side, but reality will prevail.