Initially, the tone of the poem "We grow accustomed to the Dark—" is very somber and almost scary. The poet tries to make the reader feel this sense of darkness by stressing how there is no light at all: "Light is put away." The reader is meant to feel all alone in the darkness with no other people there to help them: this is highlighted by the line "To witness her Good bye."
However, after the dark tone of the first stanza, hope begins to slowly shine through in the second stanza. As a result, the tone of the poem begins to change. Initially, the poet talks about the "uncertainty" we experience when stepping out into the darkness. Suddenly, there is a glimmer of hope as we "fit our Vision to the Dark." This could be interpreted as an indication of a more positive tone, as being able to see in the dark enables us to stand proud and "erect" instead of fearful and subdued.
At this point, the tone of the poem is no longer threatening and fearful. Instead, the tone of the poem becomes realistic. The poet does not deny that things will be difficult. Dickinson is very clear about the fact that a walk in the dark will be challenging, as some might even "hit a Tree." However, the poet does make it clear to the reader that they should keep persevering. The last line of the poem could definitely be interpreted as very hopeful: "and Life steps almost straight." One could argue that the tone of this last line is in total contrast to the darkness described in the first line of the poem.