I think that you will find different points in Steinbeck's work where many of the themes converge within one another. This makes pure distinction of themes difficult, but rather creates opportunities where different themes are present in specific moments. For example, take the exchange between Lennie and Curley's wife before she is killed. This is a moment where the theme of individual dreams versus external reality is evident in her narrative. Her desire to be more than she is and to have aspired far more than what she did is representative of this theme. At the same time, the theme of companionship and the need to have it is evident in how she yearns to talk to someone, even Lennie, about her own past and her need to find company in the silence of her own being. This same convergence is seen in Crooks' room when Lennie and he, and later Candy, all talk about their own hopes for the future in stark contrast to the grim present. The entire premise that Lennie and George share about the farm, being together in this process of achieving their dream, and how through their own shared efforts they will find their own small corner of paradise is reflective of the American Dream theme as well as the theme of companionship. The ending undercuts this moment, as Lennie's death represents, to a great extent, the fragmentation of both tenets. In these instances, specific themes coincide with one another to create a setting where Steinbeck heightens thematic complexity.
There are many themes in Of Mice and Men, the key themes are:
dreams and reality
pessimism or optimism?
protest and prejudice