There is little question that the poetry of Shakespeare's Romeo and Juliet is absolutely stellar (forgive the pun). The light/dark imagery extends from the opening prologue which mentions the "star-crossed" lovers until the end of the play. Each line is written in iambic pentatmeter, giving a melodic lilt to each passage.
When Romeo first sees Juliet, he is star-struck:
Oh, she doth teach the torches to burn bright!
It seems she hangs upon the cheek of night
Like a rich jewel in an Ethiop's ear--
Beauty too rich for use, for earth too dear!
So shows a snowy dove trooping with crows
As yonder lady o'er her fellows shows.
In this passage, Shakespeare employs simile, alliteration,and light/dark imagery to express Romeo's infatuation with the lovely Juliet. Then, when Romeo does speak to Juliet for the first time, their language is elevated to the form of a sonnet in which each speaks seven lines of the extended metaphor of his lips being pilgrims on their way to the "holy shrine" of Juliet's lips. Romeo falls immediately in love; he adores Juliet, and begs her to grant him a kiss, "lest faith turn to despair." He tells Juliet, "Thus from my lips by thine my sin is purged." [perfect meter!]
The imagery of the sonnet is that of reverent devotion; Romeo and Juliet's love is pure and holy, unlike the infatuation that Romeo has felt for Rosalind. Rosalind now becomes associated with darkness and Juliet with lightness.