It looks like you are using Fixed Page UI (User Interface)
the canvas area you show is affected by a Settings> Advanced Option
FixedPageUI [ GradientColors #ff0000 TextColor = #000000 ...
will add full red colour to all background canvas
you can add 2 or 3 colour gradients from start to finish of document
e.g. for blue to red to green it is GradientColors #0000ff #ff0000 #00ff00
or any mix you wish hexadecimal values are 00-ff for #RRGGBB
If you are referring to the background to change that color it is changed like this
FixedPageUI [ TextColor = #000000 BackgroundColor = #ff0000
thank u so much!!!