How does John Steinbeck develop his theme(s) on the nature of power?

In Of Mice and Men, Steinbeck shows us two forms of power, principally through two characters: Curley and Slim. 

Curley's power comes from his wealth and status: he's the owner's son, and for that reason, the farmhands must do as he says. However, the farmhands resent Curley, and find him belligerent and combative (Candy says that Curley's "pretty handy", meaning he likes to start fights). Curley seems well aware of this, and so he makes a point to enforce his authority whenever possible ("Well, nex' time you answer when your spoken to"). In this way, Steinbeck seems to make the point that Curley's power isn't genuine. The men do listen to Curley, as they are essentially forced to, but they have no real respect for him. 

Slim, on the other hand, gets his power from his great skill as a jerkline skinner (mule-team driver). He is a wise man who's well-respected by everyone on the farm, including Curley.

Curley said, "Well, I didn't mean nothing, Slim. I just ast you."

Slim said, "Well, you been askin' me too often. I'm gettin' God damn sick of it. If you can't look after your own God damn wife, what you expect me to do about it? You lay offa me."

"I'm jus' tryin to tell you I didn't mean nothing," said Curley. "I jus' thought you might of saw her."

In these lines, Slim rebukes Curley. We can gather from the context that Curley has (yet again) accused Slim of flirting with his wife, and this time Slim will not tolerate the accusation. Although Curley is the boss's son, the fact that Curley responds so deferentially to Slim suggests that Slim has the power here. Indeed, in this scene Slim's rebuke puts some fire in the other farmhands, who ordinarily wouldn't dare talk back to Curley:

“Why’n’t you tell her to stay the hell home where she belongs?” said Carlson. “You let her hang around bunk houses and pretty soon you’re gonna have som’pin on your hands and you won’t be able to do nothing about it.”

Curley whirled on Carlson. “You keep outa this les’ you wanta step outside.”

Carlson laughed. “You God damn punk,” he said. “You tried to throw a scare into Slim, an’ you couldn’t make it stick. Slim throwed a scare into you. You’re yella as a frog belly. I don’t care if you’re the best welter in the country. You come for me, an’ I’ll kick your God damn head off.”

Candy joined the attack with joy. “Glove fulla vaseline,” he said disgustedly.

Slim's power is also shown in another scene, where Carlson is offering to shoot Candy's dog. The old dog is in a very bad way: he's blind, missing teeth and cannot chew his own food, but Candy doesn't want to kill the dog as he's had him so long. 

“Carl’s right, Candy. That dog ain’t no good to himself. I wisht somebody’d shoot me if I get old an’ a cripple.” Candy looked helplessly at him, for Slim’s opinions were law.

In the end, it is Slim's words that convince Candy to allow Carlson to shoot the old dog. 

Slim's power has been made known to us from the beginning, when he is first introduced to us: "His authority was so great that his word was taken on any subject, be it politics or love."

Steinbeck's message, then, is that true power and true authority must be earned. Slim's power is genuine because he has earned it through his wisdom and skill. Curley does have his own form of power, but it's false and illusory, and is easily shattered when confronted by true power and authority.