This question has already been asked and answered many times here on eNotes. Here is a link for you: http://www.enotes.com/necklace/q-and-a/what-theme-short-story-necklace-also-how-does-8339
Of course, any text opens itself to a multiplicity of given themes and meanings, and this masterful short story is no exception. However, you might like to consider the topic of avarice and what the story says about this important theme. The presentation of avarice necessarily focuses on the character of Madame Loisel, who, in spite of what she has the relative comfort that she enjoys, never appears to be satisfied and is always shown to want more. We are told that things that "anotehr woman of her class would not even have noticed" ate at her soul and frustrated her. Although she has clothes to wear, food to eat and a servant to help around the house and a comfortable life, she is always plagued by the desire for more and a feeling of dissatisfaction. She is fixated on always having more and enjoying a better style of life, as her daydreams show. She always longs for what she does not have:
She had no evening clothes, no jewels, nothing. But those were the things she wanted; she felt that was the kind of life for her. She so much longed to please, be envied, be fascinating and sought after.
Her attitude when her husband receives the invitation thus supports this presentation of her character. Note how she is determined to have jewelry to go with the gown that her husband can only just afford. When she looks at the diamond necklace, we are told that her "pulse beat faster with longing." Avarice dominates her character. We can therefore say that the fate she suffers is perhaps just in terms of her determination to be unsatisfied with the comfortable life she leads, although we might feel pity at the way that she drags her poor, generous husband with her to share the same fate.