login-logo-site-icon.php

<?php

add_action( 
	'login_head', 
	function () {
		$site_icon_url = get_site_icon_url();

		if ( ! empty( $site_icon_url ) ) {
			printf(
				'<style type="text/css">
					.login h1 a {
						background-image:url("%s");
						border-radius:50%%;
					}
				</style>',
				esc_url( $site_icon_url )
			);
		}
	}, 
	100 
);