Manipulation
Letitia is the kind of woman who will get what she wants. This means she manipulates. Her particular brand of manipulation involves social relations. Through an elaborate plot, she convinces Doricourt that he truly loves her when before he did not. She also uses her charm and influence to help Lady Frances on her personal quest to social esteem. Frances just needs to hear the right things about herself, as if needing permission to be herself, in order to start rising through the socialite ranks. When Letitia recognizes this, she intercedes. She discovers that Frances' husband, Sir George, is deliberately holding her back from realizing her dreams because he's afraid she won't care for him anymore. Letitia won't stand for it and helps Frances just to prove a point to George.
Feminism
Letitia is a woman of determination. Even as a young girl, she formed an opinion about what she wanted in life and then accompanied that opinion with a plan of action for how to achieve that desire. She's a confident, capable woman who will not allow anyone else to control her. When faced with the prospect of marriage, then, she ensures that she enters into it only on her own terms. Basically, she just wants to be loved. Consequently, she doesn't rely upon anybody else to maker her happy. She sets her mind on something and then she does it, for herself. This sort of confident, strong female protagonist was rare to see in the 1780s, so Letitia is a particularly strong feminist voice.
Marriage
This play is a comedy of manners, which is basically the theatrical equivalent of a social drama. Each of the main characters -- Letitia, Doricourt, Sir George, and Lady Frances -- are concerned with the idea of marriage in some way. For Letitia, she does not want to be married to a man who loves her any less than she loves him. She views marriage as a sort of higher aspiration and consequently is unwilling to compromise any aspect of her idea of what marriage should be. Doricourt is engaged to Letitia, but he doesn't love her. In fact, he doesn't like her all that much, so the idea of marrying her has slipped into his unconscious. Sir George and Lady Frances have recently married. He is discovering that he is very insecure in himself and his abilities as a husband, so he responds by removing Frances' autonomy whenever possible. As far as Lady Frances is concerned, her marriage is a trap. She does love George and has always been faithful to him, but he has done nothing but make her life miserable in return, so she turns outward. She is more concerned about developing herself and achieving social status than improving her marriage.
Social Class
This play is set in a time when social position determined nearly every aspect of a person's life. These characters are mostly concerned with their social positioning from some angle. This is particularly true of Lady Frances and Letitia. Amidst her troubling marriage, Frances wants to develop herself. Born of noble birth, she has the advantage to naturally belong to the aristocracy, but she is insecure and lacks the confidence to really put her lineage to work. Although she doesn't personally aspire for the same high social esteem as Frances, Letitia possesses the confidence and ingenuity to advance up the ladder. She uses her skill to teach Frances how to attain what she's looking for in her own identity and in her recognized social persona.