media-gallery-widget.css

.media-widget-gallery-preview {
	display: flex;
	justify-content: flex-start;
	flex-wrap: wrap;
}

.media-widget-gallery-preview .gallery-item {
	box-sizing: border-box;
	width: 50%;
	margin: 0;
	padding: 1.79104477%;
}

/*
 * Use targetted nth-last-child selectors to control the size of each image
 * based on how many gallery items are present in the grid.
 * See: https://alistapart.com/article/quantity-queries-for-css
 */
.media-widget-gallery-preview .gallery-item:nth-last-child(3):first-child,
.media-widget-gallery-preview .gallery-item:nth-last-child(3):first-child ~ .gallery-item,
.media-widget-gallery-preview .gallery-item:nth-last-child(n+5),
.media-widget-gallery-preview .gallery-item:nth-last-child(n+5) ~ .gallery-item,
.media-widget-gallery-preview .gallery-item:nth-last-child(n+6),
.media-widget-gallery-preview .gallery-item:nth-last-child(n+6) ~ .gallery-item {
	max-width: 33.33%;
}

.media-widget-gallery-preview .gallery-item img {
	height: auto;
	vertical-align: bottom;
}

.media-widget-gallery-preview .gallery-icon-placeholder {
	box-sizing: border-box;
	display: flex;
	align-items: center;
	justify-content: center;
	max-width: 100%;
	width: 150px;
	height: 100%;
	background-color: #ededed;
	border: 1px solid #dbdbdb;
	font-weight: 600;
}