custom logo in wp-login.php

functions.php

// Add CSS within in wp-login, place in (child) theme functions.php
add_action('login_enqueue_scripts', function(){
	wp_enqueue_style('login-styles', get_stylesheet_directory_uri().'/login.css');
});

login.css

//login.css, place in (child) theme folder
body .login h1 a {
  background-image: url('path/to/custom-logo.png');
}