What is the theme of Middlemarch?

A major theme in Middlemarch is Eliot's exploration of the way the web of society is both necessary and restrictive. Eliot argues that we are not simply individuals pursuing our own desires but that what we do impacts the people around us, and their actions, in turn, impact us. Middlemarch is not about a lone hero, or a handful of lone heroes, but about the life of an entire town. For this reason, Eliot subtitled her novel "A Study of Provincial Life."
In this web of life, people are pressured to conform—and the harm of conformity becomes a second theme of this novel. Lydgate, for example, wants to do great things as a doctor/researcher, but his marriage to the very conventional Rosamond and his attempts to go his own way are both failures. One needs allies to buck convention—and a supportive wife. Lydgate's lack of understanding of the web of society and his bad marriage means he ends up selling out his talent for money, something Eliot condemns. All of society suffers when a person can't pursue their talents and ideals. Lydgate himself suffers from an empty life as he abandons his youthful hopes.
Dorothea also feels the pressure to conform—in her case to the role of the good wife in her unsatisfactory marriage to Casaubon even as she realizes he is a man of very limited intellectual abilities. He too feels he must conform to his own image as a solitary genius and rejects Dorothea's enthusiasm and desire to help him. He would rather keep up a front than admit he is behind the times in his research. Especially in the cases of Lydgate and Dorothea, the web of society traps people so that they can't fully use their gifts.