As you can see from above image, the page doesn't call css file. To fix that error on your prestashop site, open success.php (you can find this on your payumoney payment gateway folder). After that, don't forget to backup the file first.
1. Find the following code
include(dirname(__FILE__).'/../../config/config.inc.php');
include(dirname(__FILE__).'/../../init.php');
include(dirname(__FILE__).'/payu.php');
include(dirname(__FILE__).'/../../header.php');
2. Change above code into below code
include(dirname(__FILE__).'/../../config/config.inc.php');
include(dirname(__FILE__).'/../../init.php');
include(dirname(__FILE__).'/payu.php');
$context = Context::getContext();
$fc=new Frontcontroller();
$fc->setmedia();
include(dirname(__FILE__).'/../../header.php');