Greed and power
One of the main themes is what Denise learns about Paris from Octave Mouret. She learns that he has gotten extremely wealthy by using his vast network to do trade more quickly, efficiently, and for lower cost than his competitors. He uses this to drive small local businesses to the ground, because they cannot compete with his low price and wide variety. This isn't just a character, strictly speaking—it's a thematic portrait of greed and power.
Love and infatuation
The novel shows us a version of Denise who isn't closed off to Octave just because he is ruthless in business. She is also a powerful character, because by understanding the situation, she correctly avoids Octave when he comes to woo her. Now, when he grovels at her feet and begs, she can't help but fall for him, because it's a sign of her power to have a local kingpin of Paris desperately in love with her. But is it love, or is it just infatuation? The novel cleverly raises that question by suggesting that it is love (because they get married).
Class injustice
The novel opens with an undeniable portrait of class struggle. While people like Octave have enough advantages to take advantage of, many of the poor are unimaginably in need, and instead of offering better lives to his employees, Octave forces them into conflict and competition by paying them too little and keeping them desperate.
