Well, I hope you are talking about the theme of the short story written by Guy de Maupassant. If so, I would say there are a couple of ways to read the story and to understand the themes presented there.
First of all, you, of course, may read this piece as a story about greed, for the reality of Mathilde’s situation is that she is not wealthy and doesn’t belong to the social class she wants. Still, she is doing everything to be there. She lives in some kind of imaginary world, where the actual life she is living doesn’t match the ideal picture in her head. So, her illusory wealth and class are only illusions, and other people can easily see it.
On the other hand, you can have a look at this situation from the different perspective. Try to read the story as of it is written about pride. Mathilde is a proud woman; we can’t deny it. She feels herself too far beyond the tight budget circumstances she is forced to live in. To be accurate, the situation is disgusting to her. Also, after Mathilde loses her necklace, she is actually poor, her beauty fades, and she is about to learn the pride of a different sort, the pride of her work and endurance.