WordPress: Easily 'White Label' /wp-admin/

functions.php

<?php

add_action('login_head', function() {
  $custom_logo_id = get_theme_mod( 'custom_logo' );
  $logo = wp_get_attachment_image_src( $custom_logo_id , 'full' );
  if ( has_custom_logo() ) {
          echo '<style type="text/css">
          .login h1 a {
            background-image:url(' . esc_url( $logo[0] ) . ') !important;
            background-size:contain;
            width: 100%;
            height: 84px; /* edit to your specification. Default is 84px. */
          }
          </style>';
  }
});

add_filter( 'login_headerurl', function($url) {
  return esc_url( get_bloginfo('url') );
});