:root {
	--paper-padding: 4em;
}

body {position: relative;}
main {background: #ddd; z-index: 1; position: relative;}
main:before {
	content: '';
	position: absolute;
	width: calc(var(--container-width) + 2 * var(--paper-padding));
	height: 100%;
	z-index: 0;
	left: calc(50% - (var(--container-width) + 2 * var(--paper-padding)) / 2);
	box-shadow: 0px 0px 6px rgba(0,0,0,0.2);
	background: white;
}
footer {z-index: 1;}
main > * {position: relative; z-index: 1}

.site-header {box-shadow: 0px 2px 6px rgba(0,0,0,0.2);}
.site-nav {box-shadow: 0px -2px 6px rgba(0,0,0,0.2);}
.site-footer {box-shadow: 0px -2px 3px rgba(0,0,0,0.2);}