Like previously stated, the loss of freedom due to marriage, and the existence of the self-assertive woman despite of social limitation are, indeed, themes in the story.
However, upon performing a close reading you can find many more themes that equally resonate with the essence of those previously- mentioned themes.
Self-reliance- Mrs. Mallard gives more weight to personal freedom than to the joy of marriage. This is important, since the historical context of the novel is in a time when women's aim in life was to be married and be mistresses of the household. Yet, Mrs. Mallard finds no joy in this ultimate "goal" for women, and would much rather explore life alone than in the company of someone she does not love. In a modern eye this story is understandable. But, had you read this story when it first became published, you may have been somewhat scandalized.
Self-reliance is also an important theme because, from the start, Chopin warns the reader about Louise's "weak heart". And yet, far from a weakened woman, we find in Louise someone so hungry for life that, as she hears the news of her husbands death, and runs to her room to digest such news, she comes out of it described as
a goddess of Victory.
Gender roles - Brentley ruins Louise's life. He is not a bad husband. The story even says that she "may have loved him once". And, yet, he ruins her life. Why? Because the gender roles are so defined in Louise's society that women are aware at all times of how limited their lives really are. Man is king of the castle; woman is nurturer and servant. That was all that Chopin saw back in her time; to have a main character long for an independent life away from the "virtuous values" that were to "become" a woman, was simply unthinkable.
Repression- Mrs. Mallard's lack of love, passion, and motivation in her life (which is the same as to say her "married" life, as there was no other life for women except for the nunnery), leads her to repress her emotions to the point of implosion. This is what is suggested when Chopin attributes a weak heart for Louise: why would her heart be so weak when she lives in a good home, and with a good man? This is allegorical to a starved, hungry heart that has begun to die slowly. The only hope for this heart to come back was the freedom that, unfortunately could have only come from Brently's death. Since he was alive, after all, what else could that same heart do, but to die for once and for all?
If you continue to read between the lines, and draw out a good characterization of Mrs. Mallard, you will find even more themes that could very well be added to those discussed.
Themes are not set in stone by the author. You, as a reader, have the author's permission to draw out themes as they relate to you and your experience with the piece of literature. Therefore, there is NO right or wrong answer when it comes to identifying themes as long as you show evidence from the story of what those themes are, according to how you identified with the story (text to text, text to self, text to world).