Make a setting that turns cells a dark/black color and the lines white. Twitter has a setting like this, where your timeline turns dark blue and the words turn white.
Since the cells are formatted for what they will look like when printed, the default is to have white background with black text. You can make your own worksheets “Night” theme by selecting the entire sheet and then setting the background of the cells to a dark color, the font to a light color, and the cell borders to a light color. You could also apply a small dark image as the background image, which will be tiled across the entire sheet.
It would be great to hear how you think Excel should handle a Night mode for the sheet automatically. For example, if the sheet has content that is in a wide range of colors, should they automatically be adjusted to be easily visible with a dark background, even if that’s not how the author wants them to show up in the light mode?
I would like to see this implemented over the top of cell styles. Night mode could automatically darken the cell background colour (say 80%) and invert the cell's font colour. Fonts that would turn pure white should be greyed out slightly to reduce the contrast.