Symbolism, Imagery, Allegory
You probably already know the usual meanings that go with light or darkness in books and movies. Light generally represents life, goodness, mental clarity, and knowledge, whereas darkness connotes evil, death, obscurity, and ignorance. The good guy rides the white horse, the bad guy gets the black one—you know the routine.
To some extent, these traditional meanings for light and darkness are at play in The Silver Chair. When Jill, Eustace, and Puddleglum are fighting off enchantment in the Underworld and struggle to prove that something exists outside of the Queen's dark realm, they immediately reach for the image of the sun, which acts as both the actual star that provides light and warmth to Overland and becomes symbolic for mental clarity and knowledge:
"I've seen the sun coming up out of the sea of a morning and sinking behind the mountains at night. And I've seen him in the midday sky when I couldn't look at him for brightness."
Puddleglum's words had a very rousing effect. The other three all breathed again and looked at one another like people newly awaked. (12.176-177)
Puddleglum's rally here doesn't last because of the enchanting fire and because they have to draw a comparison to the queen's dim lamps, which is nothing like the sun—they are in a land of, at best, false goodness so long as the queen reigns. The dimness of the Underworld and its use of fake light contribute to the zombie-like state of the Earthmen and matches up with the evil intentions of the queen and her dark magic.
But beware: Darkness is not considered evil in all situations, and light is not a universal good for all creatures. Rilian learns from Golg that the worst possible fate the gnome can imagine is being forced to live on the surface of the earth. This is partially because of the exposure and the emptiness of the sky—the gnomes prefer the cozy, close spaces underground—but it is clear that the light of the upper world will also be too much for their eyes, which have not seen the fires of Bism in a long while and are used to the darkness of the caves.
In short, when it comes to lightness and darkness, you can use them as symbolic guides, but remember that they aren't completely, er, black and white when it comes to their meanings. And yes, that pun was totally intended.