It's simple to change your site's background settings — as long as you don't confuse them with your page background settings. Here's the difference: the Site background surrounds the Page background.
|Site Background Settings|
|Color||Click the color picker, and then select the color you want to use. If you want your background to transition between multiple colors, select Gradient.|
|Image||To change your website's background to an image, click Change, and then upload an image from your computer or select a stock image. You can also import an image from your Facebook® or Instagram® account. For more information about images, see Add image.|
|Alignment||Click the arrows to obtain the position you want for your background image.|
|Repeat||To pattern your background image vertically, horizontally, or both, use the drop-down menu to select how you want your background image to repeat.|
|Scrolling||If you want the background image to scroll in tandem with your page, select Scroll background with page.|
The change is applied to your entire Website.