According to the enotes Study Guide on "The Red Wheelbarrow," the theme of the poem is as follows:
Themes and Meanings
What “depends upon” a red wheelbarrow, white chickens, and rain? The reader is aware of the usefulness—in the case of rain, the necessity—of these things in the external world. The things referred to in the poem are also particular instances of types and classes of things—the wheelbarrow being a machine, for example, on which life also depends. Furthermore, sensations, feelings, emotions, thoughts, and ideas depend on such things. As the poet expresses it in his poem “A Sort of a Song,” “No ideas/ but in things.” The faculty of the mind that has ideas is the imagination. “The Red Wheelbarrow” is about the relationship between the imagination and reality.
The poem, then, is about how human imagination interprets what the senses relate about reality to the human mind.
So much depends upon the "so much," if you will. Whatever associations one finds in the images the poem presents, are dependent upon one's imagination. It is the imagination that sees and finds meaning in objects, glaze (the sun must now be out following a rain) and water, and chickens.
Human minds interpret what they experience through their senses and find meaning, and, according to Williams, it is the imagination that interprets and finds meaning. The other commentators above, for instance, have done just that in interpreting the poem.