The tone of a story is the way the author expresses the attitude of the story. It can change throughout the story, but there is generally an overall tone in most stories.
The overall tone in A Christmas Carol, I would say, is fairly dark, seeing as how no matter where Ebenezer Scrooge goes or looks, it makes him sad in the end. All three spirits show him fairly horrible things, except the spirit of Christmas past, but that is the past, and it's still sad, seeing as how one cannot return to the past. Not until the very end are things somewhat jovial, since Scrooge spends Christmas with the family of his poorly-paid employee.
