There are a couple of themes in this story. One is class differences. Francie's mother is a single parent who struggled to take care of her daughter to be able to send her to a prestigious school. Francie is a self-confessed slob, while her roommate is stylish and neat.
Another theme has to do with keeping secrets and not really knowing people. Francie thought she knew everything about her mother. When her mother dies, however, Francie discovers that her mother had been keeping secrets from her. Her father hadn't been killed by a bus. He was alive and living in the same city. When Francie goes to visit him, she discovers that besides the fact that he has made no effort to see his daughter, her father has other secrets.
See the link below for a good summary and discussion of themes.