In Romeo and Juliet, Shakespeare uses chiaroscuro, "an effect of contrasted light and shadow created by light" as symbolic imagery used to contrast the other dualities in the play: "love and hate"; "life and death"; and love-sickness and mating.
"Light" is mentioned 46 times in the play; "day," its synonym, is used 88 times:
Here, Montague describes Romeo not in terms of darkness (that's not very nice), but in terms of light (the lack of it) to show his melancholic love-sickness over Rosalind.
"Dark" and "darkness" are used only 9 times. Other words, though, stand for it, namely "night," used 100 times.
Earth-treading stars that make dark heaven light:
Such comfort as do lusty young men feel
When well-apparell'd April on the heel
Of limping winter treads, even such delight
Among fresh female buds shall you this night
Inherit at my house
Here, Capulet uses the whole gamut: "dark," "light," and "night" to show how "lusty young men feel" during April's mating season.
All in all, "light" and "dark" reinforce the dualities of emotional and physical love.