Symbolism, Imagery, Allegory
You can't get far in the novel without running into some references to Light and Dark—and yes, in this book they are capitalized. Which is the first clue that something's up. Light and dark often represent good and evil, but since they're capitalized in this book we know we're dealing with Good and Evil. It's like good and evil got called up from the minors.
In The Dark is Rising, Light and Dark represent two opposing forces. One of the first things Will learns about being an Old One is that knowing who's on your team and who you're fighting against is pretty important in world warfare. As Merriman explains:
"If you can accomplish that, you will have brought to life one of the three great forces that the Old Ones must turn soon towards vanquishing the powers of the Dark, which are reaching out now steadily and stealthily over all this world." (3.59)
To be clear, the Old Ones are the Light and the bad guys (like the Rider) are the Dark. Instead of just being motley crews on opposite sides, though, these folks are organized into official categories of Dark and Light, which is good since the entire world is at stake in their fight.
This whole light and dark business is a pretty classic way to represent the struggle between good and evil. It shows up in literature, movies, and visual art all the time. In this book, the capitalization reminds us that this struggle is organized and official, with two powerful teams fighting for the world.