Though Shakespeare's play is infused with delightful humor, its main theme arguably is the nature of love: who has it, who wants it, who deserves it. For example, Malvolio loves Olivia, but he will never have her and his character, "an affectioned ass" does not deserve her. As for the Duke, he happily gives up the Countess to the newcomer whe he learns that his 'boy" is a lovely maiden who will wed him. Sir Toby marries Maria for her wit, and in the end, it is only Malvolio who is unhappy, both for himself and envious of others who have fallen in love (love that is reciprocated.)
When you are reading twelfth night, there are 2 themes you need to keep in mind deception and farce.(for e.g.) Viola becoming a guy is farce, and she is decieving everybody so that is deception.