Tone in a literary work, shows the author's attitudes toward the subject and toward the audience. The tone can take on a variety of styles and may be formal, informal, sad, depressed, happy, intimate, solemn, somber, critical, sarcastic, mocking, playful, serious, ironic, condescending, or many other possible attitudes. In a work of literature we are often confronted by at least one theme, or central idea about a topic. The manner in which the author approaches the work, determines the work's tone.
One can gauge a writer's tone by studying the language, imagery and other descriptors he/she uses throughout the work. His or her approach will inform the reader whether he/she wishes one to share his or her approach.
O. Henry, through a third person narrative, adopts a sentimental tone in this story. He clearly has a fondness for the characters and he wishes the reader to share this sentiment. The language used throughout has a warm and pleasant quality, and there is hardly any reference to darkness, except of a reference to Jim's 'dark assertion', but even this is dismissed to be dealt with later.
The story is about a loving couple of diminished means, but this fact does not dampen their ardour for one another. The story focuses on Della and we learn, through her, about the depth of their devotion to each other. A few references to the text makes this quite obvious:
But whenever Mr. James Dillingham Young came home and reached his flat above he was called "Jim" and greatly hugged by Mrs. James Dillingham Young, already introduced to you as Della. Which is all very good.
Her Jim. Many a happy hour she had spent planning for something nice for him. Something fine and rare and sterling—something just a little bit near to being worthy of the honor of being owned by Jim.
Even when Della has her beautiful tresses removed, the outcome is still positive:
Out of his trance Jim seemed quickly to wake. He enfolded his Della.
I don't think there's anything in the way of a haircut or a shave or a shampoo that could make me like my girl any less.
The story explores the utterly unselfish sacrifices that those who truly love are willing to make. They are prepared to give up their most precious possessions, not for some or other reward, but because they love the ones to whom they give.
In the end it is clear that the narrator wants us to share his sentiment:
... let it be said that of all who give gifts these two were the wisest. O all who give and receive gifts, such as they are wisest. Everywhere they are wisest. They are the magi.
And, of course, we do.