This is a good question. There are several places in the story where the author give a sense of the tone. In view of this, let me give you a few examples of the tone.
First, as the story opens up, we see that the description of the night gives off a very eerie sense. The night is dark and stormy and the White's are home alone. The impression is that chaos and powers beyond their control are outside and they are safe as long as nothing is allowed to intrude into their lives. There is also mention of deserted roads, which adds to the ambiance of fear.
Second, part two of the story opens in a very different way. It is a bright winter day. This contrast also adds to the tone of the story. Something does not seem right. In other words, how can darkness give way to light that fast? Darkness lingers and is confirmed from the news of the death of Herbert.
Third, after the death of Herbert, Mrs. White asks for the paw is a frenzy, presumably to wish for her dead son. Mr. White at this point looks for the paw to undo this. There is knock on the door and it does not go away. There is fear of the unknown here and when the door is finally opened, no one is there.