What is the theme of "The Story of an Hour"?

"The Story of an Hour" is specifically about the ways in which marriage constrained nineteenth-century women from living life as they wished. Marriage was not an equal partnership but an arrangement in which, more often than not, the man was the one in charge.
The most obvious way to show this would have been to make Louise's husband out to be a tyrannical ogre figure, but Chopin is much more clever and realistic in her treatment of him. He is described as pretty normal and a decent fellow who never intended to hurt his wife. He simply assumes, as all of society did in the Victorian era, that women are meant to serve men within a marriage and have no life of their own outside the domestic sphere. Even Louise has some affection for him, which Chopin wryly alludes to: "She did love him. Sometimes." However, this is not enough for Louise, and Chopin seems to suggest it should not be for women in general.
Louise has no identity aside from "wife" until she believes her husband has been killed. With him no longer there, she realizes she can do whatever she wishes without having to run all her plans by a husband. She starts to feel an intense connection with the natural world outside her window, examining the birds in particular, evoking the notion that Louise is a bird about to be free of its cage.
Chopin hammers in the idea of marriage as existential imprisonment for women by having the story end with Louise realizing her husband survived the accident and subsequently dying from shock. Now that Louise has known true joy, having it revoked so suddenly not only triggers her heart trouble, it spiritually destroys her.

One of the themes in “The Story of an Hour” by Kate Chopin is freedom. The news of the sudden death of her husband, Brently Mallard, is given to Mrs. Mallard as gently as is possible by her sister Josephine and her husband's friend, Richards. The information is delivered in this manner because Mrs. Mallard is known to have a heart condition that is sensitive to shocking news. On receiving the news, Mrs. Mallard weeps. She is grief-stricken.
Later, after calming down a little, she retires to her room. It is while sitting in her room that she realizes the freedom that the death of her husband would bring her way. The thought possesses her so much that the words “free, free, free” escape her lips. She is excited by the thought of the new independence she will have, for with these thoughts, “her pulses beat fast, and the coursing blood warms and relaxes every inch of her body.”
She understands that her joy for her newfound freedom and independence does not mean that she hated her husband. What she hated was to live a life in which her will was constantly bent by another person. She would, from then on, live for herself. She is thrilled by the power that lies in being in control of one’s life without the interference of a spouse. She thinks of the many springs and summers yet to come that she would enjoy alone, doing whatever she wanted to do. When Brently walks into the house at the end of the story, he unknowingly tears this freedom from his wife.