.seamless-background{--bg-mobile:var(--bg-mobile-url,url(/path/to/default-bg.png));--bg-desktop:var(--bg-desktop-url,url(/path/to/default-bg-desktop.png));background-image:var(--bg-mobile);background-position:top;background-repeat:repeat-y;background-size:100%;background-attachment:scroll}@media (min-width:768px){.seamless-background{background-image:var(--bg-desktop)}}