Much of the figurative language used to depict the light and dark is in reference to the blossoming love we see between Romeo and Juliet. We see this most strongly in the balcony scene, Act II, scene ii.
For example, lines 2-3 set this idea into motion when Romeo states:
Romeo compares Juliet to the sun here as though she is illuminating his world and eliminating the dark emptiness he felt in his love for Roseline. The comparison of Juliet to light-bearing celestial bodies (sun, moon, stars, etc.) continues throughout this scene.
Two of the fairest stars in all the heaven,
Having some business, do entreat her eyes
To twinkle in their spheres till they return.
What if her eyes were there, they in her head?
The brightness of her cheek would shame those stars,
As daylight doth a lamp; her eyes in heaven
Would through the airy region stream so bright
That birds would sing and think it were not night.
For Romeo, Juliet is his "bright angel."
Likewise, Juliet enters into this same metaphor, stating:
Later in the scene, even when Juliet is questioning the logic of the emotions she feels it is this same vein of figurative language that brings her back:
It is too rash, too unadvised, too sudden;
Too like the lightning, which doth cease to be
Ere one can say 'It lightens.' Sweet, good night!
This bud of love, by summer's ripening breath,
May prove a beauteous flower when next we meet.
Much of the figurative language in parts of Act II (and actually even more so in Act I when Romeo first meets Juliet) is based around the difference between light and dark.
To me, Shakespeare is using this difference to talk about the ways in which Romeo's life has changed since he has met Juliet. Before, his life was dark. He was moping around because Rosalind did not love him.
Now that he has met Juliet, his life is lighter. He is happier than he used to be. This shows up in all of the figurative language about light and dark in this play.