How to have full screen background image with no margin?

I’m building a really simple one page site and I want a background image to cover the top portion of it. The image looked fine but it had a small margin on either side, I altered the margins to -40 on the left, top and right side which worked but only for the upper and left hand side margin. The right one doesn’t seem to want to fill the screen.

I’m using the ghost kit plugin to edit the spacing for mobile.

I must be doing something wrong but I can’t work out what it is! Can someone help me make the background image fill all the way to the edge?

Thanks ??

The page I need help with: [log in to see the link]