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.
I would rather speak of three themes, not of one. As I understand the story, it has three significant issues, which are the deceptiveness of appearances, the danger of martyrdom and the perceived power of objects. I probably should clarify my statement, so here is how I understand this headlines.
The deceptiveness of appearances is about making your life seem to be something different from what you have in reality. So, Mathilde does everything she can to appear a better person, wealthier, belonging to the high-class and so on. She doesn't ever think that her wealth and high class are merely illusions, she lives in a fantasy world, she created for herself. The deceptiveness of appearances is highlighted by Madame Forestier’s necklace, which appears to be made of diamonds but is actually nothing more than costume jewellery.
The danger of martyrdom is in the perception of Mathilde. She considers herself to be a martyr, and it leads to the unwise self-serving reactions.
And the perceived power of objects is in Mathilde’s belief, that material objects can change her entire life. Those objects, she desires most, are the necklace and the dress, and when she finally gets them, her happiness is fleeting at best.