Change CSS URL in template files

We currently have a website which uses Worldpay as its payment provider. When you make a payment it takes you to the Worldpay site and then at the end calls the file [login to view URL] file from our website. As the [login to view URL] file is called from within the Worldpay secure environment the page doesn't display properly as the css files aren't on the Worldpay server. We have uploaded the css files onto the Worldpay server and now we need someone to edit the relevant PHP files which calls the css files for the [login to view URL] file, so that they are called from the Worldpay server rather than our site. The path to the files on the Worldpay server is - [login to view URL] Where INSTALL ID is our installation ID and the [login to view URL] is the name of the file of css file we have uploaded to their server. I would suggest you go to our site and do a test transaction, add an item to the cart and go through the checkout process, select debit / credit card as the payment and fill in the details. The site is in test mode and so it doesn't actually process any card transactions at the moment. When you get to the end and are shown our site again, you will see the formatting is incorrect because the css hasn't been loaded. So essentially, this is an easy job. Find the file that needs to call the css files from the worldpay server and add the call and that is it. I have attached a full copy of all the files from the site, so you can see which file(s) to edit. We will happily share the URL of the site with anyone who sends us a message from now on and obviously the installid etc will be provided to the person who accept the job. If you need any further explanation or help then please let us know. We need this doing in the next 2-3 days. Thanks