Theme of Sacrifice in 'The Gift of the Magi'

In the short story 'The Gift of the Magi' by O. Henry, a young couple, Jim and Della, are very much in love but are struggling financially. As Christmas approaches, both want to buy a special gift for one another to express their love. Della has beautiful long hair, which she decides to sell in order to buy a platinum chain for Jim's prized possession, his watch. Meanwhile, Jim sells his watch to buy Della a set of combs for her hair. Unbeknownst to each other, their sacrifices render their gifts useless but symbolize their deep love and commitment.

What is the central theme of this story?