To adjust a css file you'll need to access your server via SFTP. You get the username . password from here:
https://www.jamroom.net/perrie/hosting/server_config/34
or:
YOUR PROFILE -> HOSTING -> DASHBOARD -> SERVER SETTINGS
That is the info you need to access your server.
With that info you are able to login and see all the files that make your site run.
Navigate to:
/YOUR SITE/public_html/skins/paPerrieNingja/css/site.css
* download that to your computer
* open the file with a text editor like notepad
* add the code to the bottom
* upload the file back to the same place
That will change the contents of the version that is on your server and will make the changes your after.
--edit--
Here's a video on how to do it:
Updating CSS files via FTP
updated by @michael: 07/03/15 11:45:47PM