Set grey Background color of Scene

Hi Forum
I would like to set a grey background color of my scenes, so the white character drawings stands out.
Somrthing like this
http://tobyshelton.blogspot.dk/2015/12/operation-got-your-6-with-penguins-of_26.html

What is the best way to do this?

Kind regards Mads

On a new layer… Use the rectangle tool to draw a bg … Fill with the grey using paint bucket.

Copy the layer

Select all the panels

In layers menu (I think) use paste layer special which pastes it on all the panels. Check the option to place the layer at the bottom.