There are a lot of good themes in this novel.
The main theme of the novel is charity. The kind of charity whereby the person is not recognized. Saint Theresa is spoken of at the beginning is one of these such people. The narrator then postulates about other people who had a positive effect on the world but nobody gave them any credit. The story of Middlemarch is an example.
The interactions of the men and women are quite different. The wives of Middlemarch are always expected to be low-keyed and submissive however they should possess talents that are treated as hobbies. Celia, Mrs. Garth and Mrs. Bulstrode are characters that are like this.
Dorothea doesn't choose stupid and trite hobbies that the other women chose. She gives herself totally to her husband's work when he is living.
Inheritance is another theme in Middlemarch, showing how how land and capitol are passed down the male lineage after death. The old patterns of patriarchy are illustrated in this novel: males having privilege/power and paternity.
Marital relationships is another theme in Middlemarch. There are three story lines, all involving different marriages. First the courship period is shown where the couples, Dorothea and Casaubon, Rosamond and Lydgate, have illusions about marriage. Their impressions are not realistic. Worst of all is their expectations that they nursed during the courtship were squashed by the actual married life.
In addition greed and the past returning to haunt the characters are other themes.
Propriety, the stifling of one's emotions and desires in order to earn or keep the acceptance of Society.