help me on themes

Man -vs- Man, or should I say Man's inhumanity toward man. This would include the novel's focus on economic status (rich/poor; landowner/tenant), and the struggle of those affected by the dust bowl to retain any dignity whatsoever.

Man -vs- Nature encompasses the Dust Bowl itself, and the far reaching consequences of being left with nothing. People didn't cause their own predicaments, they were an act of God.