With Page View set, and fixed width editor deselected, I wanted to set the background colour to make a contrast with the page colour. It changed the page colour and not the backgound.
In addition, the new colour went slightly beyond the right hand edge of the page and filled the vertical space between pages.
With fixed width editor set, in Page View the paper colour AND the backgound colour all changed to the new backgound colour setting.
This appears to be a bug unless there is interference from another setting that I am not aware of.