How does Golding use light and dark imagery to convey the deeper meanings of the novel?

One way  he uses light and dark is that the jungle is dark.  This underlies a sense of danger and the idea that in the jungle lurks evil.  In contrast, the beach and especially our initial description of the island seen through the eyes of Ralph is full of light.  

Also, the need for a fire is significant.  Fire is a symbol of light and more importantly, the fire must be made, watched over and never allowed to go out.  The boys want the fire to be seen so that they may be rescued.  Additionally it adds warmth and light in the darkness.  Fire could be read as a symbol of civilization and the fear that they might lose the fire could symbolize the fear of the wild.