A Sound of Thunder is a time travel story that conveys a theme of the importance of all events and actions. The story focuses on a single event that occurs on the time travel safari that is run by the Time Safari Company and how a small mishap, the killing of a butterfly, changed the course of evolution.
Bradbury points out in this story how the careless behavior of one of the travellers causes a shift in the course of evolution. The story also focuses on a the fact that the group who takes the time traveling trips is a group of rich businessmen and Bradbury is saying that the fact that this type of company exists, and that people have nothing else to do with their money, is vulgar and obscene.
Bradbury, like other Science Fiction writers, such as H.G. Wells, writes about the fear of technology going too far. In this story, I think Bradbury is commenting on how technology has been misused. The technology that has allowed these time travelers to take a trip with Time Safari is clearly an expression of materialism and over consumption on the highest level.
For their personal pleasure, these people, rich people who have nothing else to do, alter the course of human evolution and nearly destroy human existence.
Therefore, the theme that Bradbury is trying to convey has to do with the Butterfly Effect and Chaos theory, which states that a small difference can have an enormous impact on future events.
"The butterfly effect is a theorem of chaos theory that small variations in the initial conditions of a dynamical system can produce large variations in the results. A Sound of Thunder. In that story, a time traveller accidentally steps on a butterfly in the distant past, causing broad changes in the present."