Light and dark, and white and black are important symbols in Oliver Twist. Notice how often Oliver’s trapped someplace dark? Notice how the sun always comes out? No matter how dark things get for Oliver (metaphorically speaking), you know things are going to brighten up eventually.
Some examples: Oliver, the child of light, is locked into an involuntary apprenticeship with a coffin maker, Mr. Sowerberry. Oliver is asked to join the funeral processions as a paid mourner (although Mr. Sowerberry gets paid for his services, as an apprentice, Oliver doesn’t see a penny of it). Oliver’s commitment to life is contrasted with the darkness and death that surrounds him. The parish authorities had originally wanted to apprentice Oliver to a chimneysweeper – a job that would have caused Oliver to spend however long he survived blackened with soot and ash. When Oliver is arrested for picking Mr. Brownlow’s pocket, the officer who makes up a name for Oliver unconsciously picks an appropriate one: "White." Oliver is as pure as the driven snow, while all of the areas of London associated with the criminal class are stained black.
Or take a look at these two passages: "[…] the heavy bell of St Paul’s tolled for the death of another day. Midnight had come upon the crowded city. The palace, the night-cellar, the jail, the madhouse; the chambers of birth and death, of health and sickness; the rigid face of the corpse and the calm sleep of the child – midnight was upon them all" (46.4). This is another moment of social leveling through the use of darkness: Dickens lists a lot of contrasting places – palaces, night-cellars (bars that didn’t have liquor licenses), jails, madhouses, etc. – and also juxtaposes a lot of extremes: birth and death, sickness and health, corpses and sleeping children. Time passes for all of these extremes, and it’s equally dark at midnight whether you live in a palace or a madhouse.
Now compare that with this passage about light:
"The sun – the bright sun, that brings back not light alone, but new life, and hope, and freshness to man – burst upon the crowded city in clear and radiant glory. Through costly-coloured glass and paper-mended window, through cathedral dome and rotten crevice, it shed its equal ray. It lighted up the room where the murdered woman lay. It did." (48.2)
The sun, like midnight, is a social leveler – it shines equally on everybody, whether through expensive stained glass, or through a window mended with paper or duct tape. He even uses the word "equal" – the sun "shed its equal ray" – after juxtaposing a bunch of extremes ("costly-coloured glass" and "paper-mended window," and "cathedral dome" and "rotten crevice"). And no matter how dark it was, you can bet the sun is going to come out. Dickens even makes the parallel between "light" and "life" explicit here – the sun "brings back" both.