Love
Despite the complicated way in which the girls both arrive at love, the story is essentially a love story. The theme of love also shows that the heart cannot really be led in a specific direction; the heart wants what it wants. Rose intends to fall in love with Simon Cotton, and to make him fall in love with her, because he is the heir to the Cotton family fortune, which she believes will help drag the Mortmains out of poverty as well. However, although she does win Simon's love, it is not true love. She falls in love with Neil, who reciprocates her feelings, and the couple sacrifice both their intentions for their lives and their relationships with their families for love.
Similarly, Simon Cotton is in love with Rose, and intends to marry her, but finds himself inexplicably drawn to Cassandra. This is an example of the sub-theme of forbidden love, as is the relationship between Neil and Rose; Cassandra is aware that she should not be falling in love with her sister's fiance; Simon is aware that he should not be falling in love with his fiance's sister, yet both cannot stop their feelings developing however inappropriate they know them to be.
Ultimately Cassandra also shows that love is patient, choosing to wait until Simon returns from his trip to America before admitting the strength and depth of her love for him.
Genteel Poverty
The Mortmains are a genteel, respectable family, yet they are considered "on their uppers", which means that they are approaching poverty faster with every day that goes by. This was somewhat of a theme socially in England in the 1930s and is a theme seen often in books about the upper middle classes at the time. Another example is E.Nesbitt's classic children's novel The Story of the Treasure Seekers, which chronicles the fall in fortunes of genteel families living in London. The Mortmains are forced to sell furniture in order to put food on the table, and it seems to Rose and Cassandra that in the face of their father's complete lack of literary output, the only solution to their poverty is going to be marrying well.
Propriety
The novel also shows the importance placed upon propriety in the upper middle classes at the time. This is seen early on in the novel, when the Cottons and the Mortmains are initially getting to know each other and in fact getting along very well. It is considered unseemly for a woman to flirt, and if she does, she should at least be both expert and subtle about it. Rose is neither of these things and her obvious, amateur flirting is a social repellent to the Cotton brothers, who immediately consider her vulgar. The theme also deals with the way in which the actions of one member of a family can signal social death for the family as a whole; the brothers do not just cut off all ties with flirty Rose, but with the entire family, because of her behavior.
