What are the themes of Shoeless Joe?

I would assert that the thematic link present is that reality is largely defined by the ability to see what can be as opposed to what is.  The driving force in the novel is Ray's desire to transform his reality through imagining what can be as opposed to what is.  Mowing down his crops to spend three years in building the field represents a sense of imagination that allows him to transform his reality.  In an almost dream like pursuit of Salinger, Ray again seeks to make his reality in alignment with his own sense of imagination.  To observe the players on the field, one has to learn to "see." 

This ability to see is the possession of imagination, something that Mark lacks.  It is also something that Ray's brother lacks until Ray teaches him how to use his imagination.  The expansion of a moral imagination allows reality to mirror dreams, as evidenced in how Ray's brother and father emerging with connection.  The end of the story, where Ray becomes the active force behind others' dreams and their imagination, helps them to transform their reality, as well.