What is the primary theme of 'Of Mice and Men'?

Of Mice and Men can be interpreted in any ways. The title comes from Robert Burns's poem, "To a Mouse," in the line, "The best laid plans of mice and men often fo awry." The most common interpretation I've heard is that mice refers to weak people, such as Lennie, Crooks, and Candy, and men refers to strong people, like George and Slim. In all essence, the book is a story of weak and strong people, and follows the quote from the poem exactly.