Certainly debate will exist over what the "main" theme is of a work of literature. However, one theme you can definitely identify and talk about is the use and abuse of power in this book.
One relationship you will definitely want to examine is that of the boss and his workers. The first time the boss meets George and Lennie it is clear he has power over his workers, as they need employment to survive, and he has the ability to hire and fire at will. We can see this in his suspicion about Lennie and why he doesn't answer any questions and he checks very carefully why they finished their last job. He then says to George and Lennie: "'All right. But don't try to put nothing over, 'cause you can't get away with nothing. I seeen wise guys before...'" Clearly the future of the workers lies in the boss' hands - he will not be messed around, and if George and Lennie don't work hard or cause any problems they will be fired.
Curley feels that he has power over the men at the farm due to his status as the son of the boss. This is why he struts around importantly, as his word to his father could result in one of the workers getting fired. He is full of self-importance and we can see in his fight with Lennie (although of course he loses it) that he feels he has more power than the workers. He says to Lennie: "'Come on, ya big bastard. Get up on your feet. No big son-of-a-bitch is gonna laugh at me. I'll show ya who's yella." This shows he does not expect any oposition and takes his power for granted.
You also have to examine the relationship between George and Lennie, because George definitely has power over Lennie, though he, perhaps unlike others, uses his power for good, to help Lennie and eventually to save him from a nasty end. George says about Lennie that "he could have broke every bone in my body but he never lay a hand on me", thus showing although Lennie is so much more physically powerful than George, it is George who has the power over Lennie, and it is to George that Lennie looks when he needs comfort and advice.
Lastly, Curley's wife has power over Lennie in her ability to manipulate and persuade him. Although George has told Lennie not to talk to her, she persuades him to stay and talk anyway: "'What kinda harm am I doin' to you?". Curley's wife obviously misuses her power for her own ends, although she doesn't realise the folly of spending time with Lennie until it is too late.
Hopefully this will give you enough to talk about in your work! Good luck!
It is difficult to pick up just one theme that Steinbeck is trying to portray in the novel because there are several important ones. One of them is the conflict between idealism and reality, George and Lennie have an idealized way of dreaming about the farm that they might own together that would solve their problems and leave them in a good place. This of course doesn't survive when George kills Lennie because he knows that his dream can't survive reality.
One of the other main themes is each of the character's desperate struggle against loneliness and the struggle to find friendship and companionship in the extremely cold and heartless world the characters live in.
there are many but i think that desire and innocence are the most important.
desire to be free and support themselves "live offa fatta da land" and how they talk about growing food and selling all the excess
innocence because Lennie is innocent, hes still innocent when he kills them mice, puppy, and Curley's wife