In regards to a theme for "The Rocking-Horse Winner", I was going to say money does not buy happiness. Would that be a good theme?I would have to use evidence to back up my thoughts
Yes, you can use the theme “money does not buy happiness” or you can also make it a little more sophisticated. For example, you could also say that we should not be so driven by success that we forget what’s really important in life. As the enotes theme page notes, there is a "disparity between Paul's generosity and his mother's greed" (enotes themes).
Hester is a “woman who was beautiful, who started with all the advantages, yet she had no luck” (http://readytogoebooks.com/DHL-rock1.htm)
Hester desperately wants, but does not have, success. Yet she pushes to get it so much that she loses track of what is happening with her own son. Paul risks his life and his health to try to get her the information she needs, betting on and winning on horses. She tells her son that luck is more important than money.
"Oh!" said the boy. "Then what is luck, mother?"
"It's what causes you to have money. If you're lucky you have money. That' s why it's better to be born lucky than rich. If you're rich, you may lose your money. But if you're lucky, you will always get more money."
"Oh! Will you? And is father not lucky?"
"Very unlucky, I should say," she said bitterly. (http://readytogoebooks.com/DHL-rock1.htm)
It is this fascination with luck, success, and money that leads to her downfall. Paul wants desperately to help her, but she seems to have no interest in helping him and does not even realize what he is doing. She is at a party when he turns ill, thus symbolizing her greed and selfishness.