"O, then I see Queen Mab hath been with you" (1,4,58)
"In bed asleep while they do dream things true" (1,4,57)
Romeo is talking about how dreams can be real or true and Mercutio says that they are a lie and reflect the individuals desire or passion.
"I fear too early, for my mind misgives Some consequence yet hanging in the stars." (1,4, 114)
"Through lovers' brains, and then they dream of love; On courtiers' knees, that dream of cur'sies straight; O'er lawyers' fingers, who straight dream on fees." (1,4,76)
In this scene dreams are present in this scene because Romeo and Mercutio are debating about the fact if dreams are true or not. Well Mercutio explains to him that people dream of things they aspire to have and not what is true.
The theme in this scene is that people dream of things that they want and aspire to have.
Shakespeare is trying to show the fact that Romeo loves Juliet and desires love. It might not become true or it might but it will come with a big consequence.