WordPress/wp-content/themes/twentynineteen/sass/site/header/_site-featured-image.scss

// Featured image styles

.site-header.featured-image {

	/* Hide overflow for overflowing featured image */
	overflow: hidden;

	/* Need relative positioning to properly align layers. */
	position: relative;

	/* Add text shadow to text, to increase readability. */
	text-shadow: 0 1px 2px rgba(0, 0, 0, 0.35);

	/* Set white text color when featured image is set. */
	.site-branding .site-title,
	.site-branding .site-description,
	.main-navigation a:after,
	.main-navigation .main-menu > li.menu-item-has-children:after,
	.main-navigation li,
	.social-navigation li,
	.entry-meta,
	.entry-title {
		color: $color__background-body;
	}

	.main-navigation a,
	.main-navigation a + svg,
	.social-navigation a,
	.site-title a,
	.site-featured-image a {
		color: $color__background-body;
		transition: opacity $link_transition ease-in-out;

		&:hover,
		&:active,
		&:hover + svg,
		&:active + svg {
			color: $color__background-body;
			opacity: 0.6;
		}

		&:focus,
		&:focus + svg {
			color: $color__background-body;
		}
	}

	.main-navigation .sub-menu a {
		opacity: inherit;
	}

	/* add focus state to social media icons */
	.social-navigation a {
		&:focus {
			color: $color__background-body;
			opacity: 1;
			border-bottom: 1px solid $color__background-body;
		}
	}

	.social-navigation svg,
	.site-featured-image svg {
		/* Use -webkit- only if supporting: Chrome < 54, iOS < 9.3, Android < 4.4.4 */
		-webkit-filter: drop-shadow(0 1px 2px rgba(0, 0, 0, 0.35) );
				filter: drop-shadow(0 1px 2px rgba(0, 0, 0, 0.35) );
	}

	/* Entry header */
	.site-featured-image {

		/* First layer: grayscale. */
		.post-thumbnail img {
			height: auto;
			left: 50%;
			max-width: 1000%;
			min-height: 100%;
			min-width: 100vw;
			position: absolute;
			top: 50%;
			transform: translateX(-50%) translateY(-50%);
			width: auto;
			z-index: 1;

			@supports ( object-fit: cover ) {
				height: 100%;
				left: 0;
				object-fit: cover;
				top: 0;
				transform: none;
				width: 100%;
			}

			/* When image filters are active, make it grayscale to colorize it blue. */
			.image-filters-enabled & {
				filter: grayscale(100%);
			}
		}

		.entry-header {

			margin-top: calc( 4 * #{$size__spacing-unit});
			margin-bottom: 0;
			margin-left: 0;
			margin-right: 0;

			@include media (tablet) {

				margin-left: $size__site-margins;
				margin-right: $size__site-margins;
			}

			.entry-title {

				&:before {
					background: $color__background-body;
				}
			}

			/* Entry meta */

			.entry-meta {

				font-weight: 500;

				> span {

					margin-right: $size__spacing-unit;
					display: inline-block;

					&:last-child {
						margin-right: 0;
					}
				}

				a {

					@include link-transition;
					color: currentColor;

					&:hover {
						text-decoration: none;
					}
				}

				.svg-icon {
					position: relative;
					display: inline-block;
					vertical-align: middle;
					margin-right: 0.5em;
				}

				.discussion-avatar-list {
					display: none;
				}
			}

			&.has-discussion {

				@include media (tablet) {

					.entry-meta {
						display: flex;
						position: relative;
					}

					.entry-title {
						padding-right: calc(1 * (100vw / 12) + #{$size__spacing-unit});
					}

					.entry-meta .comment-count {
						position: absolute;
						right: 0;
					}

					.entry-meta .discussion-avatar-list {
						display: block;
						position: absolute;
						bottom: 100%;
					}
				}
			}
		}
	}

	/* Custom Logo Link */

	.custom-logo-link {

		background: $color__background-body;
		box-shadow: 0 0 0 0 rgba($color__background-body, 0);

		&:hover,
		&:active,
		&:focus {
			box-shadow: 0 0 0 2px rgba($color__background-body, 1);
		}
	}

	/* Make sure important elements are above pseudo elements used for effects. */
	.site-branding {
		position: relative;
		z-index: 10;
	}

	.site-featured-image .entry-header {
		position: relative;
		z-index: 9;
	}

	/* Set up image filter layer positioning */
	.site-branding-container:after,
	.site-featured-image:before,
	.site-featured-image:after,
	&:after {
		display: block;
		position: absolute;
		top: 0; left: 0;
		content: "\020";
		width: 100%;
		height: 100%;
	}

	/* Background & Effects */
	/* Shared background settings between pseudo elements. */
	background-position: center;
	background-repeat: no-repeat;
	background-size: cover;

	/* The intensity of each blend mode is controlled via layer opacity. */

	/* Second layer: screen. */
	.image-filters-enabled & .site-featured-image:before {
		background: $color__link;
		mix-blend-mode: screen;
		opacity: 0.1;
	}

	/* Third layer: multiply. */
	/* When image filters are inactive, a black overlay is added. */
	.site-featured-image:after {
		background: #000;
		mix-blend-mode: multiply;
		opacity: .7;

		/* When image filters are active, a blue overlay is added. */
		.image-filters-enabled & {
			background: $color__link;
			opacity: .8;
			z-index: 3;

			/* Browsers supporting mix-blend-mode don't need opacity < 1 */
			@supports (mix-blend-mode: multiply) {
				opacity: 1;
			}
		}
	}

	/* Fourth layer: overlay. */
	.image-filters-enabled & .site-branding-container:after {
		background: rgba(0, 0, 0, 0.35);
		mix-blend-mode: overlay;
		opacity: 0.5;
		z-index: 4;

		/* Browsers supporting mix-blend-mode can have a light overlay */
		@supports (mix-blend-mode: overlay) {
			background: rgba($color__background-body, 0.35);
		}
	}

	/* Fifth layer: readability overlay */
	&:after {
		background: #000;
		/**
		 * Add a transition to the readability overlay, to add a subtle
		 * but smooth effect when resizing the screen.
		 */
		transition: opacity 1200ms ease-in-out;
		opacity: 0.7;
		z-index: 5;

		/* When image filters are active, a blue overlay is added. */
		.image-filters-enabled & {
			background: mix($color__link, black, 12%);
			opacity: 0.38;

			@include media(tablet) {
				opacity: 0.18;
			}
		}
	}


	::-moz-selection {
		background: rgba($color__background-body, 0.17);
	}

	::selection {
		background: rgba($color__background-body, 0.17);
	}
}