The theme of “The Necklace” is that we ought to be grateful for what we have rather than constantly wishing for more. Madame Loisel pities herself immensely because of her lack of wealth and her personal feeling that she was “born for every delicacy and luxury.” Instead of being surrounded by opulence—“exquisite pieces of furniture supporting priceless ornaments”—she is compelled to make do with her marriage to a man with a good job, a servant to help her with the most menial wifely duties, and a comfortable home with food on the table.

Despite all that she does have, she “suffers endlessly” because she wants so much more. Madame Loisel is unable to appreciate a husband who would go to great lengths to secure an invitation to a party just to make her happy, who would give up the money that he was saving for something for himself so that she could have a new dress for this one night, and so on. He is grateful for what they have, and she is not.

The necklace she borrows from her friend, Madame Forestier, embodies extent of Madame Loisel’s desire for luxury, at any cost. The necklace is a symbol of her lust for wealth: she cannot recognize that what she already has in her life has true, real value, just as she cannot recognize the worthlessness of the necklace, which is not actually a diamond at all. She thinks that she knows what is truly valuable, but she does not. The irony of the necklace’s actual cheapness underscores the idea that we should be grateful for and recognize the good things in our lives rather than being so focused on acquiring more.

The main themes of “The Necklace” first and foremost, in my opinion, is Class Conflict as Madame Loisel, a member of lower class strives to appear to be in a higher class than she actually is, which causes conflict, but it also hints at another theme of Appearances and Reality.  Madame Loisel is determined to make herself appear to be at a higher level on the social ladder than she actually is; however her desire to attain this goal leads to a display of Generosity on the part of Madame Forestier as she loans a very expensive necklace to the Greedy Madame Loisel who looses it.  The necklace actually seems to be a symbol of her life, the insincerity of her character, and how her greed leads to her ultimate misery in life because she is forced to give up her own goals in life to repay Madame Forestier for the necklace she has lost, effectively ending her feeble climb up the social ladder.  Irony is best demonstrated in the fact that she wastes her life to pay for a cheap copy of the original necklace, while she herself is nothing more than a cheap copy. Check the links below for more information.

The theme of the short story, "The Necklace" is to be happy with who you are and what you have.  Madame Loisel lived in the middle class society but longed to be a member of high society.  She wanted to live above her means and would do whatever it took to get there.  Her husband provided the stepping stone to enter the world of the upper class by getting an invitation to a prominent reception.  From there she talked her husband into giving her the money to have a wonderful dress to fit in with them and then the final highlight was to borrow a wonderful diamond necklace from Madame Forestier to wear to the reception.  The ironic situation is the source of her joy, the necklace, becomes her downfall when she loses it and has to work the next ten years of her life to replace it.  She not only loses her middle income status but falls into poverty and spends the rest of her life suffering for her dreams.  The symbolism of the necklace is social acceptance into a world where she longs to be, but doesn't belong. 

Enotes has an excellent response to your question at the URL below. The necklace itself represents the theme of appearances versus reality. “While sufficiently beautiful to make Madame Loisel feel comfortable during the ministerial ball, the necklace is actually nothing more than paste and gilt. Thus, it is not the reality of wealth or high social class that is important for Madame Loisel, just the appearance of it.” As the discussion on enotes points out, other themes involve conflict of class (rich vs poor) and greed and generosity.  The irony in the story is that Madame Loisel ruins her life in trying to compensate for the lost necklace, when in fact the necklace was not worth anything to begin with.  As for symbolism, it resides in the title which also carries the theme:  the necklace symbolizes the difference between appearance of  and reality, a wealth that is empty, having no real value.