pure javascript to add css in a page with cakephp syntax

 <script>  
   document.write("<style>body { background-image: url(<?php echo $this->Html->url('/img/bg.jpg'); ?>); background-size: cover; background-repeat:no-repeat; }</style>");  
   </script>  

use this code in to add any css to current page with cakephp syntax to call the image path