inline-external-link-style.php

<?php
// loosely based on a selector from https://davidwalsh.name/external-links-css
wp_add_inline_style(
	'mytheme-style',
	sprintf(
		'a[href*="//"]:not([href*="%1$s"]) {
			padding-%3$s: 17px;
			background: url("%2$s") center %3$s no-repeat;
			background-size: 13px;
		}',
		esc_attr( wp_parse_url( get_site_url(), PHP_URL_HOST ) ),
		esc_url( get_theme_file_uri( 'assets/img/external-link.svg' ) ),
		( is_rtl() ? 'left' : 'right' )
	)
);