RickyXO This is very easy. You have to first know the url of the css e.g http://rickyxo.xtgem.com/Css/rickyxo.css. All you have to do is;

Get into your website's panel,

Click on the File browser,

Then click on Upload file,

Then input the css' url in the upload from url and it will automatically appear in your file browser. Just link it to your website and it's there. The same applies to pages.