OK, this is where you get started in running your own website:)
You will need to download a copy of the stylesheet to your home computer, add the line above, and upload it back to your website server.
To do this you will need two things: an FTP "client" and a good text editor.
Filezilla and Notepad++ are two good ones (there are others too), and are free from
www.sourceforge.net .
Download and install those (they have instructions to walk you through the process), and come back when you are done or if you need help.
Then you can get to editing.