Themes of pride, dissatisfaction, and greed are all clearly developed in Hester, Paul's mother. She is also a terribly selfish woman, totally incapable of loving anyone, including her son. Her heart is cold. Even when Paul dies, his mother does not feel grief or loss. We have reason to believe she will continue spending, and wasting, whatever money she gets. There will never be enough money for her.
A second theme involves sacrifice. Paul loves his mother and knows how miserable she is because they are poor. He determines to get money for the household, to be "lucky" as his father had not been. When Paul discovers he can pick the winners in horse races, he becomes obsessed with winning more and more money for his mother, but it is never enough. He works harder and harder, growing frail and weak, until he dies. He sacrifices himself for his mother's happiness, a gift she surely does not deserve and one he surely had no responsibility to give her.
Another theme develops from Paul's need for love and recognition. He is a child who is neglected and emotionally abused. His mother never listens to him; he plays no role of importance in her life. He dies trying to please her. Perhaps then she would love him.
Try not to mix up the defintions between subject and theme. A subject is a word that describes what the story is about. A theme is a central message or idea that the story creates in a complete sentence.
I have previously answered questions on D. H. Lawrence's masterful short story "The Rocking-Horse Winner."
Please take a look at http://www.enotes.com/rocking-horse-winner/q-and-a/rocking-horse-winner-what-tone-theme-setting-89071