The Martian Chronicles
The defining theme is the progress of colonization: how people from another land - in this case, planet - go to another place and claim it as their own. In doing so, they are infringing on the another race's territory and culture, eliminating the native peoples in the process. This story is most familiar to American readers in terms of the European conquest of the Americas, and Bradbury draws clear analogies between his story of Martian colonization and the real history of the American frontier.
Related to the major themes are minor themes: the hubris of human achievement, in the belief that great deeds must be recognized by others in order to be validated; the desire for freedom which often motivates settlers to leave their homes; the power of nostalgia on people, especially those who have been cut off from their past in such a powerful manner.
The mood of these stories is often elegiac, evoking a sense of loss and solemnity. Part of this comes from injustices done to Mars and Martians; another part of it comes from the emotions settlers experience when leaving their homeland to live on an alien frontier. There is also a strong thread of wryly ironic humor: sometimes it's a bitter irony, other times it's a strong sense of absurdity at a particular situation. Often, the humor is dark, complementing the elegiac mood instead of contrasting against it