This very short story tells of one hour lived in the life of Louise Mallard, who has just been told that her husband had died. Instead of being overwhelmed with grief, Louise has an ironic reaction: she feels free and happy. Chopin describes Louise as feeling "Free! Body and soul, free!" Louise did feel sad that her husband died--he had been kind and she "had loved him, sometimes," but felt marriage to be stifling and repressing, so looked forward to the years ahead that "would be hers and hers alone."
There are a couple possible themes to this story. One is repression in marriage. In Chopin's time period, women were born and bred to be married, and it was supposed to be the ultimate form of happiness. That was what life was for. But, in this story, Chopin is asserting that it can repress women and stifle their individuality, and that not all women are happy in marriage. Another potential theme is that of irony, or, the message that we shouldn't assume things. Just because her husband had died, we shouldn't assume it was all bad for Louise. Just because she was shocked and died at the end, of what the doctors concluded was "joy that kills," doesn't mean that she died for joy when her husband walked in. Appearances and stereotypes can be deceiving, so, don't assume things.
I hope those thoughts help a bit; I provided a link below to some other possible themes that might work also. Good luck!