Find and replace is the key tool...
Make sure you are on the "site" screen
Like nippi said, "Back-Up your site" & select the pull down "entire local site" and copy and paste the code with out the change in the box next to "search for", also set search for to source code...
In the replace with window paste the same code in and insert the
<link rel="stylesheet" href="yourstylesheet.css" type="text/css">
part in the code
Then hit find all and replace all