I mean add this to the wp_head hook:
<link rel="stylesheet" href="http://www.w3schools.com/lib/w3.css">
Or better yet, upload it to your server, and link to it:
<link rel="stylesheet" href="http://yourwebsite.com/w3.css">
To make sure you’re calling the right URL, enter it into your browser and make sure it takes you to the CSS (yourwebsite.com/w3.css for example).