This is a classic, if somewhat obvious, motif to use in characterizing the battle between the strikers and the looters. However, light and dark do more than just stand in for the good guys and the bad guys here. The oncoming darkness hints at approaching disaster and the increasingly rapid decline of society. It represents not just the looters themselves, but the disastrous effects of their regime.
We kick things off with a creepy twilight scene that really sets the stage for impending doom and disaster:
The light was ebbing and Eddie Willers could not distinguish the bum's face....But from the sunset far at the end of the street, yellow glints caught his eyes, and the eyes looked straight at Eddie Willers, mocking and still – as if the question had been addressed to the causeless uneasiness within him. (220.127.116.11)
At one point Dagny and Owen Kellogg set out to find help for a broken-down train. They literally walk away from the light, which represents civilization here, into darkness, seeming to travel both back in time and to some sort of alien world:
Then she found herself watching the light on the ties under her feet, watching it ebb slowly, trying to hold it, to keep seeing its fading glow, until she knew that the hint of a glow on the wood was no longer anything but moonlight. She could not prevent the shudder that made her turn to look back. The headlight still hung behind them, like the liquid silver globe of a planet, deceptively close, but belonging to another orbit and another system. (18.104.22.168)
The light and dark symbolism are sometimes flipped around in the book. Sometimes the strike itself is tied to twilight and darkness in positive ways, and other times the looters are associated with light in very negative ways. Take this scene during the launch of the John Galt Line:
The cliffs ahead were a bright, liquid gold. Strips of shadow were lengthening the valley below. The sun was descending to the peaks in the west. They were going west, and up, toward the sun. The sky had deepened to the greenish-blue of the rails, when they saw smokestacks in a distant valley. (22.214.171.124)
In this scene the twilight is cast positively and is directly linked to the "blue-green" of Rearden Metal, one of the most positive things in the book. The scene of the train going up toward the sun is also a heroic image. In a way, the train is defiant in the face of darkness, proceeding along in spite of it, so it has a positive rather than an adversarial relationship with twilight and night. Another positive connection between darkness and the strike is the blackout of New York city at the end. This may seem like a terrible event, and it is, but the blackout also signals that Galt's quest is complete. He and his strikers can now start planning how to rebuild the world.
Light is rarely linked to the looters, but when it is, it takes on a rather sinister aspect. In this scene, light is harsh, highlighting the bad situation in which Dagny finds herself:
There was a beam of white light beating down upon the glittering metal of a microphone – in the center of a glass cage imprisoning her with Bertram Scudder. (126.96.36.199)
For the most part, light and darkness are used in the novel to represent good and evil, the strikers and the looters, in the traditional sense. The instances where the typical light and dark associations are reversed are worth noting, though.