site-logo-block-style.php

<?php
/**
 * Register block styles.
 *
 * @return void
 */
function prefix_register_block_styles() {
	$site_icon = get_site_icon_url();

	if ( $site_icon ) {
		register_block_style(
			'core/site-logo',
			array(
				'name'         => 'mobile-site-icon',
				'label'        => __( 'Mobile Icon Only', 'agency-fifty-three' ),
				'inline_style' => '@media (max-width: 551px){
					.wp-block-site-logo.is-style-mobile-site-icon .custom-logo-link {
						background-image: url(' . esc_url( $site_icon ) . ');
						width:45px; height:45px;
						background-size: contain;
					}
					.wp-block-site-logo.is-style-mobile-site-icon img {
						display:none;
					}
				}',
			)
		);
	}
}
add_action( 'init', 'prefix_register_block_styles' );