You write your main style sheet as an external file, lets say we name it basestyle.css and place it in the css folder. Now we can call it for every page we want to use it on like this:
Code:
<link href="http://www.somesite.org/css/basestyle.css" rel="stylesheet" type="text/css" />
Make it the first call and follow it with a call to the css file for that page.
This has nothing to do with javascript. It's just normal HTML/CSS.