Add primary button styling to WordPress 'Use as Featured Image' Button

wp-featured-image-button.php

<?php
/**
* Adds a primary button styling to the "Use as Featured Image" link within
* the media uploader.
*
*/
class GS_Featured_Image_Button {
  
  public function __construct()  {  
    add_action('admin_head-media-upload-popup',array($this, 'load_scripts'));
  }


  public function load_scripts( $hook_suffix ){
    ?>
    <!-- GS FEATURED IMAGE BUTTON -->
    <script type="text/javascript">
      jQuery(document).ready(function($) {

        function windowSetup() {
          // Checks if window type is set as "image".  This is not present in the normal post uploader.
          var window_type = $('#sidemenu li a').attr('href');
          var is_image_window = window_type.indexOf("&type=image");

          // Applies style changes only if tests indicate that current window is for Featured Images
          if ( is_image_window != -1  ) { 
            $('#media-upload').addClass('featured-image-window');
            $('.wp-post-thumbnail').addClass('button-primary');
          }
        }
        interval = setInterval(windowSetup,500);

      });
    </script>
    <!--/ GS FEATURED IMAGE BUTTON -->
    <?php
  }

}
$GS_Featured_Image_Button = new GS_Featured_Image_Button;