I like the coments of the previous poster, but those answers sound as much like morals as they do themes. As I understand the terms, a moral is a lesson that you're supposed to learn from reading a story (e.g. the fable about the tortoise and the hare might be told to teach a young reader that "slow and steady wins the race," i.e. that hard work and persistence will bring you success in the end). Themes, on the other hand, are more generally abstract ideas and are often framed in terms of opposiitions or pairs, such as "Self and others" or "Individual versus society."
I would say, then, that Guy de Maupassant's story "The Necklace" might have the theme of "honesty and deceit" or the theme of "appearance versus reality." These are abstract terms, of course, and need to be explained further by identifying specific elements in the story that reflect or reinforce these abstract terms.
Personally, I prefer the open-endedness of themes over the restricted wording of morals. Morals often sound to me like plattitudes; we've heard some of these items so often that they really don't seem to have much meaning left in them.
In a nutshell, 'Satisfaction comes from within, not without.' Another might be 'Be content with what you've got; more will never be quite enough.'
The restlessness and yearning of Mrs Loisel is in direct contrast with her husband's good-natured acceptance of his station in life and the little pleasures it brings. While Mr Loisel appreciates a good stew, his wife bemoans her lack of luxury and the social recognition she cannot attain. Even when they receive an invitation to the gala and she buys a fitting dress, a flower corsage won't do - she must have jewels. Her appetite for acquisition is insatiable and, ironically enough, ends up "consuming" her happiness and youth in an irretrievable way.