One theme in Of Mice and Men is friendship. This is shown by George and Lennie's loyalty to each other, even when they get angry at each other.
Another theme in Of Mice and Men is racism. Curley's wife telling Crooks that she could have him killed just because she was white and he is black is racist. He's also treated differently than the other men at the ranch.
Trust is also seen in the story. Lennie has almost childlike faith in George, trusting him to take care of all of his needs. Lennie also trusts that George will get him a rabbit someday.
We see pride in Of Mice and Men. This is especially shown in Crooks, who keeps everything he has clean and tidy-even if he doesn't have much.
Mental Illness is another theme in the story. While we are never told directly what is wrong with Lennie, it is obvious that mental illness is a part of Lennie's character.
Hope is the last and biggest theme in Of Mice and Men. George and Lennie hope to one day have their own ranch. Lennie hopes to have a rabbit. Candy hopes to join them. The whole story is based on hope for a better future.