If you position the background cornfield at the bottom of the page, there will be no background color showing below it in any monitors, and only above in large monitors.

background-position: bottom center;

In spite of the fact that it would be easier to get a good result by blending the sky into a blue background-color, you might want to position the image at the top center and fade/blur the bottom of the grass into a solid green background-color, because the bottom of the page is less prominent than the top and I think you would want to control the exact appearance of the top of the screen most.