Scrooge's office is dark because he constantly tries to save money in being stingy with candles and coal. But Dickens is also using a classical binary opposition (light/dark) to symbolize good/evil. Binary oppositions have been a staple in most movements of literature: Male/female, Public/private, Light/dark, Majority/minority. (With the emergence of Modernism and particularly Postmodernism, these oppositions began to be questioned and deconstructed as a result of the inequality of privileging one term over another - such as Male/female.)
So the binary opposition is something theorists have used to note symbolism but also for purposes of criticism. In this case, the use of light and dark to symbolize good and evil is a classic, traditional technique; not some unfair privileging of light. Therefore, Scrooge, being evil or indifferent, is associated with the darkness. Those expressing love and joy are associated with light. Note the description of Scrooge's nephew in Stave One. He is joyful; therefore, associated with warmth and light:
He had so heated himself with rapid walking in the fog and frost, this nephew of Scrooge's, that he was all in a glow; his face was ruddy and handsome; his eyes sparkled, and his breath smoked again.
Just as light symbolizes goodness and darkness symbolizes evil, warmth is synonymous with love, cold with hate. Since Scrooge hates the joy associated with Christmas, he hates the bright attitudes associated with it. He thinks such boisterous behavior is silly and pointless because it profits no money. By opposition, he prefers "his usual melancholy tavern." Scrooge's attitude reflects his surroundings (and vice versa). He is cold (unfriendly) and prefers a quiet, hard-working man to one who is bright, cheerful and sociable.
The most obvious reason Scrooge likes the darkness is because it saves him money on having to buy candles, and Dickens spares no expense in letting the reader know that Scrooge is a miser, for he tells us that Scrooge is " a tight-fisted hand at the grindstone,. . . a squeezing, wrenching, grasping, scraping, clutching, covetous old sinner." All of these adjectives allude to Scrooge's relentless miserly love for money.
However, one could argue that Scrooge likes the darkness because darkness represents evil, and Scrooge is characterized as evil in "Stave I," for many times Dickens tells us that Scrooge "growls" his responses suggesting that he is some sort of creature; another example is found when Dickens describes a blind man's dog pulling his master into a doorway to avoid contact with Scrooge and wagging its tail as if to say that "no eye at all is better than an evil eye, dark master." The "evil eye" being Scrooge's.
He likes the darkness because he doesn't have to pay to light his house but it also shrouds him from being seen for what he truly is, ugly and mean. The darkness is symbolic of Scrooge's ways.