Putting others ahead of yourself
For much of her life, Mrs. Eliza Touchet put the life and well-being of her cousin, famous author William Ainsworth, above her own life and interests. Ainsworth, who was a particularly mean and cantankerous man, demanded a lot from Touchet and didn't allow her to have a life of her own.
After having put the needs of her cousin above her own for most of her life, Touchet decided to change. Because Ainsworth began declining and demanding less from her, she took up things that she liked doing: writing, spending time with her friends and family, and abolitionism. And though she never regrets her life, she discovers that one can—and should—find balance in their lives. Otherwise, someone is liable to be burnt out and unhappy because putting someone else in front of you can lead to neglect and a poor sense of self-worth.
Standing up for what is right
Another important theme in the novel is standing up for what is right. Despite having just escaped slavery in Jamaica and immigrated to England, Andrew Bogle decides to do what is right and testify in the so-called "Tichborne Trial," even though he doesn't have to. If he didn't testify, his life would be easier, and he wouldn't draw attention to himself—attention which he didn't want and could ill afford to have. In the end, despite the inherent risk that came with testifying, Bogle was rewarded for doing what was right and given long-lasting friendships.
The damaging effects of sexism
The damaging effects of sexism are perhaps the most important theme in the novel. In the world of The Fraud, which is set in 1860s Victorian England, a time when men ruled the world and women had few rights to speak of, Eliza Touchet tried to buck those trends. As a result of this patriarchal society, women weren't taken seriously and weren't given the autonomy to explore their rights and their interests. Eliza, who has been subject to rampant sexism throughout her life, is one of few not to be constrained by sexism. She pursues what she wants (including abolitionism) when she wants it. And the world is better for it, showing the importance of having autonomy for women.