How does John Steinbeck reveal the theme of discrimination in his novel "of mice and men". answer using direct and relevant evidence from the novel

The stable buck at the ranch, Crooks is also the only black man in the novel. A proud and bitter man, Crooks has a cynical intelligence and a contemptuous demeanor that he uses to prevent others from inevitably excluding him because of his race. His defensive manner fades, however, once Lennie behaves kindly toward him, and he even considers helping Lennie and Candy with their plan to buy land until the threats by Curley's wife force him back into his normal combative posture. Crooks's isolation is based on race but it reflects the isolation that all characters feel in the story.