themes-wordpress/dalston/block-extends/extend-media-text-block.css

/* Media & Text Inset Borders */

.wp-block-media-text.is-style-inset-borders {
	position: relative;
}

.wp-block-media-text.is-style-inset-borders:before {
	content: '';
	position: absolute;
	top: 10px;
	right: 10px;
	bottom: 10px;
	left: 10px;
	border: 1px solid #FFFFFF;
	z-index: 1;
}

.wp-block-media-text.is-style-inset-borders.is-selected:before {
	content: inherit;
	display: none;
}

.wp-block-media-text.is-style-inset-borders.is-selected .block-library-media-text__media-container:before {
	content: '';
	position: absolute;
	top: 10px;
	right: 10px;
	bottom: 10px;
	left: 10px;
	border: 1px solid #FFFFFF;
	z-index: 1;
	border-right-width: inherit;
}

.wp-block-media-text.has-media-on-the-right.is-style-inset-borders.is-selected .block-library-media-text__media-container:before {
	border-left-width: inherit;
	border-right-width: 1px;
}