What is the theme of A Doll's House?

One major theme of the play is that women must be the social and legal equals of men; otherwise, not only their independence but also the happiness of all is compromised. Torvald does not see his wife, Nora, as his equal. One indicator of this is the nicknames that he has for her: "the squirrel," "little lark," and so on. By the end of the play, Nora has realized—based on Torvald's horrific response to the knowledge that she took out a loan and forged her father's signature to do so—that her life has been without meaning and that she does not truly know who she is. She says,

I passed from father's hands into yours. You arranged everything according to your taste; and I got the same tastes as you; or I pretended to—I don't know which—both ways, perhaps; sometimes one and sometimes the other. When I look back on it now, I seem to have been living here like a beggar, from hand to mouth. I lived by performing tricks for you, Torvald. But you would have it so. You and father have done me a great wrong. It is your fault that my life has come to nothing.

As a result of Nora's feeling that she's never really had a life or identity that belongs to her, she abandons her family: her husband and children. This is a pretty shocking turn of events, one that absolutely astonishes Torvald, especially because he claims that he has forgiven her for her "want of principle" (which he obviously only does because Krogstad has forgiven the debt). Society's refusal to allow women their own identities and legal decisions has resulted in the unhappiness of an entire family, and this could have been avoided if Nora had been treated as an equal with rights and ideas of her own.