Wallace Stevens' poem "The Idea of Order at Key West" is extremely complex, and critics have written a great deal about the poem without ever achieving a definitive interpretation. The basic narrative is of a woman walking by the sea and singing. One recurring theme is that the woman's song is intentional and has a certain order, as does all human language, including, of course, Stevens' poem itself. The sea also makes beautiful sounds, though. Thus we cannot say that beauty of sound, in poetry or song, is due exclusively to human intention at the point of creation. However, it may be that the "rage for order" of the human narrator makes over a non-ordered sound of the sea into something ordered like human song. Thus the theme is the relationship between beauty and order.
Stevens' long poem "The Idea of Order at Key West" is, indeed, complex. The answer provided above does a good job to refine the poem to its primary fixations. We can add also that Stevens was interested in exploring metacognition - thinking about thinking - and specifically, he was interested in exploring the way that language both limits and creates our view of the world.
Reality is shaped by language, yet language is shaped by reality. The relationship is mutual and cyclical. Crucially, this relationship mimics the relationship between Man and the Universe. (We can use any terms here which are expressive of the individual's relationship to the world into which he/she is born.)
There is a question of truth within this cycle: How can we capture any truth in a language that is purely descriptive? Can we say that this singer sings the truth if it is only a truth of language, of created words and sounds?
Is the song and its power only an external reality? Or does the power of the song lie in one’s ability to transform it into something personal?