Custom Sheet Size - Change Grey background outside grids

[aperrot]

Hi,

I'm using the custom sheet size property and would like to keep the white background for the grid area but change the grey one outside the grids (to another color or even a picture. How would you do that? If the only way to do it is using a theme, can you please explain how to achieve this using a theme (meaning which function to call and/or css to use)? 

Thx:)