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.