How do I change the sheet color in Numbers 3.x? Right now the sheet color is white. But it is hard on my eyes. How do I change the sheet  color to black? I'm using a Numbers 3.x in OSx 10.9.3
This is currently not possible. The closest workaround as far as I'm aware is to fill the area with a locked rectangle of your desired colour.

You can provide feedback to Apple regarding this at the following page:


https://www.apple.com/feedback/numbers.html