themes-wordpress/quadrat/sass/templates/_header.scss

.wp-site-blocks {
	position: relative; // This is needed so that the polygon is stretched across the whole site.
	 .site-header:before {
		content: "";
		background-color: var(--wp--custom--color--tertiary);
		position: absolute;
		bottom: 0;
		top: 0;
		left: 0;
		right: 0;
		z-index: -1;
		@include break-small-only() {
			clip-path: polygon(0 0, 100vw 0, 100vw 50vw, 52vw 83vw, 0 20vw);
		}
		@include break-small() {
			clip-path: polygon(0 0, 100vw 0, 100vw 37vw, 52vw 70vw, 0 5vw);
		}
		@include break-large() {
			clip-path: polygon(13vw 0, 100vw 0, 100vw 16vw, 50vw 51vw);
		}
	}
}