Word Template - Can't remove background color from default layout

Hi there guys,

I'm using a Word template with Office 365, and it has a small shaded square at the top left of every page.

I want to get rid of it, but there is no way to select it or manage the color using custom themes.

It's almost as if it's in the background, but even changing the page background does nothing to this part of the document.

When I apply gridlines, this square of colour also covers the gridlines - so I find it really strange.

Does anyone have any idea why this particular part of the document can't be changed?