I would say that the overall mood of Ray Bradbury's "A Sound of Thunder" is suspenseful and intense. Both of those feelings are given to the reader because Bradbury makes sure that the reader feels all of the tension that Eckels feels.
For example, when the story first begins Eckel's first question is "Does this safari guarantee I come back alive?" Wait, what? The reader immediately begins to question exactly why a time safari would be that dangerous in the first place. The answer is equally suspenseful.
"We guarantee nothing,"
That's just plain ominous sounding. The paragraph that follows helps set a suspenseful tone with certain words. There is consistent repetition of the words "burning" and "death."
Eventually the reader learns that Eckels is going to hunt a T. rex. That's a scary dinosaur. It's always the man eater from stories, so the fact that Eckels is going to face off against one is also suspenseful. The reader might think that Eckels will be okay because this is a time safari; it's a for profit business. People shouldn't be dying. But the company has Eckels sign a waiver.
"A Tyrannosaurus Rex. The Tyrant Lizard, the most incredible monster in history. Sign this release. Anything happens to you, we're not responsible. Those dinosaurs are hungry."
The reader is keenly aware of the fact that once the safari begins, there is no guarantee that anybody makes it out alive. The tension just keeps building from there until the climactic scene where Eckels confronts the T. rex. He freaks out and the text changes to very clipped dialogue. It reads quickly and heightens the level of suspense that the reader is experiencing.
"Why, why," Eckels twitched his mouth. "It could reach up and grab the moon."
"Sh!" Travis jerked angrily. "He hasn't seen us yet."
"It can't be killed," Eckels pronounced this verdict quietly, as if there could be no argument. He had weighed the evidence and this was his considered opinion. The rifle in his hands seemed a cap gun. "We were fools to come. This is impossible."
"Shut up!" hissed Travis.
"Turn around," commanded Travis. "Walk quietly to the Machine. We'll remit half your fee."
"I didn't realize it would be this big," said Eckels. "I miscalculated, that's all. And now I want out."
"It sees us!"
"There's the red paint on its chest!"