Background at the either sides.

Sorry you’ll need to describe more about what you want & provide more information about your theme.

Uh, I just see a black background on that example site.

In general, though, if you want your “background” to be an image, you set the CSS for the body element. You can read about it here. Some themes may have an option that allows you to set the background image, or to add custom CSS.

ok.here is another example : https://www.skidrowgames.net/age-of-empires-iii-complete-collection-multi5-prophet.html . please notice the two sides of the page.