No, this is strictly JavaScript. You can't do this with CSS. Although the technique used to include the javascript file works much like including a CSS file. Here's a code sample of including a javascript file:
<script type="text/javascript" src="/yourdirectory/yourfile.js"></script>
This is way better than using PHP's include(). Also, you don't need to keep adding carriage returns to these files since the browser is the only one that will be interested in the file's contents. And the less data you have in these files, the quicker they will transfer to your web users.
I HIGHLY recommend you look into how javascript and CSS works. How they're used. And how you actually use them. Feel free to view the source of a number of web sites and see how they make it all work.