+1 This would be a huge feature:
1) I don't know where I'm supposed to be looking in the Evernote Windows UI, a grey background would simplify the screen visually.
2) I just captured text from a website that had white letters on a black background. I thought I had an empty note until I accidentally selected and found the text in reverse. With a non-white (grey) background, its easier to see where a note begins and ends.
3) The white is hard on my eyes, especially at night, so I try not to use Evernote after 4:30 PM, or whenever the Sun sets up here...