What is the theme of the novel Pride and Prejudice?

Summary:
As in most of Austen's writing, there is an overt theme of true love overcoming all obstacles. However, underlying this somewhat sentimental depiction of romantic love is an astute and at times scathing criticism of social injustice and gender inequality in Regency England, which, it can be argued, is the true theme of Pride and Prejudice.
Explanation:
A common theme in Jane Austen's novels is the topic of true love winning over social conventions, differences in class and social status and - on the character level - major errors in judgment. Pride and Prejudice (1813), which is arguably Austen's most famous and most beloved novel, is no exception here.
While the two main characters, Elizabeth Bennet and Mr. Darcy, both belong to the so-called landed gentry (a class of landowners who lived off the rental income of their properties), Lizzy's family is depicted as socially (read: financially) inferior to Darcy and the Bingleys and is therefore treated with much disdain and condescension by these characters.
On the interpersonal level, both Mr. Darcy and Lizzy have to overcome grave errors in judgment (pride and prejudice) and grow as a person before they are allowed to live "happily ever after".
Minor Characters as Social Critique:
An technique often used by Austen is to center her social criticism around her minor characters. Pride and Prejudice laments the reality for the era's women whose only option to provide for their own futures was often to "marry well" regardless of affection. So while Lizzy and Darcy are allowed their fairy tale ending, others are not so lucky. To Lizzy's great dismay, her best friend Charlotte Lucas, who has neither Jane's beauty nor Lizzy's charisma and wit, is forced to marry the insufferable social climber Mr. Collins to achieve a financially secure future. Another example is the importance of a woman's reputation as addressesd in the elopement of Lydia Bennet with the morally corrupt Mr. Wickham, which - were it not for Darcy's interference - would have ruined the family's reputation, destroying with it any chance of marriage for each of the other Bennet sisters.

There is not one theme in Pride and Prejudice but several. However, one of the most important is the triumph of love over social convention. The society depicted by Jane Austen is rigidly hierarchical, with everyone expected to know their place and act accordingly. When it comes to getting married, wealth and social class are considered the most important things; love doesn't enter into it. So despite the growing love that Lizzie and Mr. Darcy feel for each other, they have to overcome the enormous social barriers that exist between them. Though Lizzie's father is a gentleman, he's not quite in the same league as Mr. Darcy and his family. Nor for that matter is Jane in respect to the super-snobbish Bingleys. She too must triumph over the conventions of English rural society if she's to follow her heart and marry the man she loves.