How does Dickens present positivity and happiness in A Christmas Carol?

In A Christmas Carol, Dickens shows two perspectives on positivity and happiness.
There is, initially, Scrooge's perspective as he interacts with much more positive characters throughout the text. To his nephew, Fred, and his fiancee, he expresses disdain for their love and affection in spite of their poverty, because, as he claims, money is the only thing of importance and love will not sustain them. To Tiny Tim and Cratchit, he expresses that their happiness is misguided in light of their fortunes. Scrooge's overall demeanor and attitude belies his thoughts of happiness as unnecessary and unimportant, something of a frivolity.
At the end of the novel, Scrooge's attitude comes in line with the author's, and with reality—that happiness is the light which brings us through hard times, and which strengthens us in spite of our maladies and misfortunes. The story expresses that, while happiness does nothing in and of itself to aid us in our situations, it empowers us to bear through those events.

Dickens presents positivity and happiness through the symbols of light and heat. Scrooge, for example, "had a very small fire" and he forces his clerk to work in a "dismal little cell" with a fire so small "that it looked like one coal." Scrooge is miserable and unhappy, and so he is associated with darkness and chill. Think about how we might describe someone who is callous or even cruel as "cold" and someone who is happy and welcoming as "warm." When Scrooge's nephew, Fred, arrives, he is said to be "all in a glow" with "eyes [that] sparkled": he is associated with light and warmth, as opposed to his relative.
Further, when the narrator describes the people outside, who are preparing for the Christmas holiday, there are a great many references to light and heat because everyone is happy and anticipating their lovely tomorrow. People run around "with flaring links, proffering the services to go before horses in carriages" to lead others through the fog safely. A group of workers "had lighted a great fire in a brazier, round which a party of ragged men and boys were gathered: warming their hands and winking their eyes before the blaze in rapture." Even their hard work and likely difficult lives are eclipsed by their happiness in this moment. The shops are "bright" and "holy sprigs and berries crackled in the lamp heat of the windows." Everything is light and bright and warm because the people are happy, unlike Scrooge.

One way that Dickens presents positivity and happiness in A Christmas Carol is through his portrayal of the Cratchit family. Though dirt-poor, and forever in the midst of adversity, they still retain a remarkably positive outlook on life. Their modest, overcrowded home is a place of love, laughter, and warmth, despite their poverty and the chronically poor health of Tiny Tim. Their sunny outlook on life is epitomized by Bob Cratchit, a good, kind-hearted man whose fundamental decency belies his humble station in life and the appalling treatment he regularly receives at the hands of his skinflint employer.
Contrast that with Scrooge. He has all the money in the world, and yet he leads a lonely, miserable life in his large, drafty house. The stark contrast between Scrooge and the Crachits shows that, for Dickens, happiness and positivity have nothing to do with how much money you have. Instead, they come from the heart, from a loving, caring attitude toward other people.