what are the major contrasts of the 12th night and how does each enhance a theme?
A good question. There are many contrasts in this play, so many that it may only be possible to sketch out the major ones.
Consider the contrast between Malvolio and Toby Belch. The first is an upright and uptight servant; the second is a drunken slob of a relative. The two end up in conflict, and though Sir Toby is an open fool, Malvolio is shown to be more of one. Both show the theme of self-deception.
Olivia and Orsino are in contrast. The beloved and the lover, the pursued and the pursuer, the female and the male, they both show extremes of relationship to love (reluctance and over-indulgence).
Throughout the play, there is conflict between appearance and reality, and between play/mockery and reality. These relate to the overall themes of the play of celebration (during festivals you play at being other things).