How does the marriage between Elizabeth and Darcy exemplify the themes of both love and marriage in the novel?
Love and marriage is one of the central themes of the novel and you are right to identify the relationship between Darcy and Elizabeth as being one of the main ways through which the author explores this theme. However, to fully develop this focus on love and marriage it is well worth examining the foru courtships that lead up to marriage to identify Austen's preoccupations with marriage: Charlotte Lucas and Mr. Collins, Lydia and Wickham, Jane and Bingley and finally Elizabeth and Darcy. You also will want to consider the examples of marriages that we are presented with: Mr. and Mrs. Bennet and Mr. and Mrs. Gardiner.
However, focussing on Darcy and Elizabeth, both have to overcome personal failings to arrive at love and marriage, in the form of his pride and her prejudice. Through confronting and overcoming these character failings they change considerably, and particularly in the last third of the novel, we witness a great change in their characters, with Darcy losing his cold reserve and pride (as observed in Lizzie's first trip to Pemberley) and Elizabeth losing some of her prejudice towards Darcy and discovering his true character. Compared to Jane and Bingley, theirs is a marriage of opposites, with her liveliness and pleasure in idiosyncracies finding a happy match with his decency and gravity. They help moderate each other, and their mutual knowledge of the other's weak points, the novel suggests, makes their marriage the strongest of all.