dynamic-login-logo.php

<?php
function sideroad_login_logo() {
  $ratio = (get_custom_header()->height / get_custom_header()->width);
  $width = 320;
  $height = ($width * $ratio);
?>
  <style type="text/css">
  #login h1 a, .login h1 a {
    background-image: url('<?php header_image(); ?>');
    width: <?php echo $width; ?>px;
    height: <?php echo $height; ?>px;
    background-size: <?php echo $width; ?>px <?php echo $height; ?>px;
    background-repeat: no-repeat;
    padding-bottom: 30px;
  }
  </style>
<?php }
add_action( 'login_enqueue_scripts', 'sideroad_login_logo' );
?>