WP_Theme_JSON::append_to_selector( string $selector, string $to_append )

Source

File: wp-includes/class-wp-theme-json.php

private static function append_to_selector( $selector, $to_append ) {
		$new_selectors = array();
		$selectors     = explode( ',', $selector );
		foreach ( $selectors as $sel ) {
			$new_selectors[] = $sel . $to_append;
		}

		return implode( ',', $new_selectors );
	}