Is the story mostly about Della and Jim?
In one word. Yes. The story is mostly about Jim and Della. They are a poor couple and when Christmas rolls around, they are at a loss of what to do on account of their finances. They deeply love each other, but they do not know how to express that love. So, Della concocts an idea. She will sell her hair and buy her love, Jim, a chain for his watch.
In the meantime, Jim sells his beloved watch to buy combs for Della's hair. Without them knowing, they enact something beautiful. They offer costly gift at their own expense. In other words, they gave up something they held precious and gave it away to benefit another person. By doing so, they proved to be wise. They showed great generosity. More pertinently, they had the spirit of the ancient Magi who came to the baby Jesus and offered gifts. Here is how O. Henry ends the short story:
The magi, as you know, were wise men—wonderfully wise men— who brought gifts to the newborn Christ-child... Being wise, their gifts were doubtless wise ones. And here I have told you the story of two children who were not wise. Each sold the most valuable thing he owned in order to buy a gift for the other. But let me speak a last word to the wise of these days: Of all who give gifts, these two were the most wise.