XF 1.5 Import custom CSS?

I'm trying to add a custom font for an icon to my XenForo forum, but without success.
I've tried to add it to index.php, AutoLoader.php, and many others. All ending with errors.
How can I add this link to any file without the forum giving me errors?
<link href="https://file.myfontastic.com/tNUdzoCmmxBeH9y4Bsfit/icons.css[/URL]" rel="stylesheet">