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:
Away from the light steals home my heavy son,
And private in his chamber pens himself,
Shuts up his windows, locks far daylight out
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.