The use of light and dark reveals information about the blossoming love between Romeo and Juliet on multiple levels. We see this expressed most clearly in the balcony scene (Act II, scene ii).
At the most superficial level we see Romeo standing in the dark of the orchard and Juliet standing upon the well lit balcony. Visually, this emphasizes that the two are on opposite sides of their families' feud. Juliet, a Capulet, should be opposed to Romeo, a Montague, just as light is opposed to dark.
On another level, we see also see that one cannot exist without the other. Light cannot exist without darkness and vice versa. We are beginning to learn in this scene that Romeo and Juliet each feel they cannot live without the other.
As Romeo states several times through out this scene, Juliet is the "light of his world," or "the center of his universe." In fact, he comes Juliet to several light-bearing celestial beings (the sun, the stars, angels, etc.), all showing us his belief that she is, in fact, heavenly.