What does dickinson mean beyond the literal meanings of the words when she says "we grow accustomed to the dark"? With this interpretation in mind, consider the lines "and so of larger-Darknesses - / Those Evenings of the Brain-". To what might these lines refer?

When looking at the lines from the poem that you need to analyze keep in mind that darkness = uncertainty. When you read the poem overall it gives a feeling of a hopeful person, finding their way through something. The something could be a personal matter or about life in general. "We grow accustomed to the dark" could mean we all find our way eventually through things in our lives and learn to adjust and work through adversity. "And so of larger-Darknesses / Those Evenings of the Brain ... " this could be referencing an even larger obstacle or one simply within the mind.