There is not just one theme for the play The Doll's House. It is a complex story about the complicated financial and personal problems of married couple.
One theme of the play is perception vs. reality. Torvald perceives Nora to be a silly woman who only thinks about spending money and being pretty. In reality, she is a shrewd schemer who does what she has to do to protect her family. Likewise, Dr. Rank seems to be only a very good friend to both Torvald and Nora, when in reality he is deeply in love with Nora.
Another theme that goes along with the first is truth vs. lies. Nora's lies, though meant for good, only cause her heartache and disappointment.
There are other themes, but I won't list them all for you here. You can read an excellent article here at eNotes at the link in the resources section.