You can look at the inverse of power in this novel to demonstrate the ways that power is a theme.

George, Lennie, Crooks, and Candy are all relatively powerless. They own nothing. They have no property. They toil and scrape to build another man's fortune. That is why the dream they share has such great appeal. In the dream, they have the power to say, "I don't want to work today, so I'm not going to work."

Candy also feels he has no power. Like the dog, he thinks that his injury makes him weak and no longer needed. He worries he will be put out to pasture, since he can longer work and has outlived his usefulness on the ranch. There are more characters without power than with it.

You should also consider the power Slim has (given all others look up to him) and the power Curley has (because everyone is afraid of him, except Slim). Outside of that, you could look at the power the dream has for both Lennie and George.