What is the theme in Sweet Thursday by John Steinbeck?

Change is at the heart of this story.

From the beginning of the novel change is what creates the story. The war brought upon change to the people of Cannery Row and they were forced to make changes to their way of life and business. When Doc comes back all he wants is normalcy but that is the last thing he gets, things have changed too much for him to simply jump back into his old life. The people in Cannery Row have had enough change that they just want things to stop changing and they are focusing all their energy on stopping change.