what is the theme of this story

You might consider social class as a central theme in the story. Works of Literary Realism often focus on the theme of social class, and "The Necklace" is certainly an example of this. Mme. Loisel's greatest concern is her own social class, especially the way she is perceived in society in virtue of her appearance and attire. It is her focus on social class that causes her to borrow a necklace to wear to a party to which she and her husband have been invited; in an ironic twist, this very necklace results in them becoming even lower in social class when they lose it and must work to pay for a replacement.