How is the theme of social class presented throughout the novel?

Social class plays a huge role in this novel.  All of English society in Jane Austen's time and in her books is based on social class.  The Bennet's are not poor, but they are not wealthy and are in a position to lose their home to a relative, Mr. Collins.  

Society is organized according to where you fall on the social ladder.  When Darcy meets Elizabeth, he is repulsed by his feelings for a young woman whose family is not only rather common, but not wealthy.  They have no impressive family connections either. Technically, Elizabeth is not eligible to be a possible wife for Darcy who is far above her on the social ladder.

Another example of social class dominating the novel, is Mr. Collins constant fawning all over Lady Catherine DeBourgh.  Mr. Collins brags constantly about Rosings, Lady Catherine Debourgh's beautiful home as if it should be worshipped.

The fact that Darcy cannot shake his feelings for Elizabeth is troubling for him.  He tries desperately to rid himself of his feelings for someone who is far below him socially.  So, in the end, when Darcy and Elizabeth do get together, it is a triumph for love over social class and class structure.  That is what Jane Austen loves to celebrate in her novels, because, she herself, was a victim of social class prejudice in her own love life.