It is actually possible, although you have to copy a existing note that already has a background color
http://www.christopher-mayo.com/?p=459
This stopped working in 5.91 (it still doesn't with 5.95, I checked)
Maybe because of this it stopped working
https://discussion.evernote.com/topic/88637-the-evernote-editor/
Maybe it has something to do with layout changes (new checkbox icon, new icons in the toolbars, the white background seems a little lighter (?) )
I'm staying on 5.8.8/staying off 5.9.x + versions until evernote fixes this