first

wp-content/themes/twentynineteen/sass/media/_captions.scss

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+.wp-caption {
+	margin-bottom: calc(1.5 * #{$size__spacing-unit});
+
+	&.aligncenter {
+
+		@include media(tablet) {
+			position: relative;
+			left: calc( #{$size__site-tablet-content} / 2 );
+			transform: translateX( -50% );
+		}
+
+		@include media(desktop) {
+			left: calc( #{$size__site-desktop-content} / 2 );
+		}
+	}
+}
+
+.wp-caption img[class*="wp-image-"] {
+	display: block;
+	margin-left: auto;
+	margin-right: auto;
+}
+
+.wp-caption-text {
+	color: $color__text-light;
+	font-size: $font__size-xs;
+	@include font-family( $font__heading );
+ 	line-height: $font__line-height-pre;
+ 	margin: 0;
+ 	padding: ( $size__spacing-unit * .5 );
+	text-align: center;
+}

wp-content/themes/twentynineteen/sass/media/_galleries.scss

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+.gallery {
+	display: flex;
+	flex-flow: row wrap;
+	justify-content: center;
+	margin-bottom: calc(1.5 * #{$size__spacing-unit});
+}
+
+.gallery-item {
+	display: inline-block;
+	margin-right: 16px;
+	margin-bottom: 16px;
+	text-align: center;
+	vertical-align: top;
+	width: 100%;
+
+	// Loops to enumerate the classes for gallery columns.
+	@for $i from 2 through 9 {
+		.gallery-columns-#{$i} & {
+			max-width: calc((100% - 16px * #{ $i - 1 }) / #{ $i });
+
+			&:nth-of-type(#{$i}n+#{$i}) {
+				margin-right: 0;
+			}
+		}
+	}
+
+	&:last-of-type {
+		padding-right: 0;
+	}
+}
+
+.gallery-caption {
+	display: block;
+	font-size: $font__size-xs;
+	@include font-family( $font__heading );
+	line-height: $font__line-height-pre;
+	margin: 0;
+	padding: ( $size__spacing-unit * .5 );
+}
+
+.gallery-item > div > a {
+	display: block;
+	line-height: 0;
+
+	// Accessibility
+	box-shadow: 0 0 0 0 transparent;
+
+	&:focus {
+		box-shadow: 0 0 0 2px rgba( $color__link, 1 );
+	}
+}
+

wp-content/themes/twentynineteen/sass/media/_media.scss

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+.page-content .wp-smiley,
+.entry-content .wp-smiley,
+.comment-content .wp-smiley {
+	border: none;
+	margin-bottom: 0;
+	margin-top: 0;
+	padding: 0;
+}
+
+embed,
+iframe,
+object {
+	max-width: 100%;
+}
+
+.custom-logo-link {
+	display: inline-block;
+}
+
+.avatar {
+	border-radius: 100%;
+	display: block;
+	height: calc(2.25 * #{$size__spacing-unit});
+	min-height: inherit;
+	width: calc(2.25 * #{$size__spacing-unit});
+}
+
+svg {
+	transition: fill $icon_transition ease-in-out;
+	fill: currentColor;
+}
+
+/*--------------------------------------------------------------
+## Captions
+--------------------------------------------------------------*/
+@import "captions";
+
+/*--------------------------------------------------------------
+## Galleries
+--------------------------------------------------------------*/
+@import "galleries";