In, "A Sound of Thunder," what theme does Bradbury reveal through Eckles' careless personality/actions, and the story's main conflicts (internal and external)?

In the short story "A Sound of Thunder," Ray Bradbury uses Eckels' careless personality and actions, as well as the narratives internal and external conflicts, to emphasize the theme that all actions--no matter how seemingly inconsequential--have an enormous impact across time.
The story follows a hunter named Eckels, living in the year 2055, who pays $10,000 in order to travel back to the Jurassic era to hunt a Tyrannosaurus rex. Despite the fact that Eckels is clearing warned by his guide that he must take care not to disturb too much of "history" in order to avoid setting off a cataclysmic chain of alterations, Eckels does not abide this advice. Terrified by the dinosaur, Eckels stumbles off the path and, in doing so, inadvertently makes a terrible mistake: he steps on a butterfly, killing it.
When Eckels returns to modern times, huge changes have occurred as a result of the butterfly's death. The results of an important election have altered, and the English language has shifted in both sound and spelling. 
Thus, Bradbury is highlighting the nature of the "butterfly effect"--a component of chaos theory that suggests that "small variations in the initial conditions of a dynamical system can produce large variations in the results." We must be attentive to what we do--to all of our words, actions, and choices--because it has significant consequences for the future. Our decision-making abilities and behaviors can alter the course of history and, thus, must be executed with great care.
Additionally, the external conflict of the narrative--the creation of time travel itself--and the internal conflict--whether such a tool should be used--also has implications in this theory. It is clear that Bradbury is questioning the evolution of technology; in this case, it is technology itself--and the hedonistic choices of people--that result in terrible ramifications for the human race.