Off-page background colour

Hi,
I’m looking for a way to change the background OUTSIDE the page defined in Page dimensions.
If change the page background I only change the background within the page dimensions.
This can be annoying when presenting a mobile app on a screen when a page-wide element on the mobile screen has the same color grey as the background around the mobile screen. And therefore I would like to switch the background around the screen (to black for example).
Regards,
Igor