CSS is for styling HTML. It will not add content to your site, though it will style all the content of your site.
For example you could control all the content between <p>This is a paragraph</p> tags with this:
Then all paragraph text would be red and bold. On every page where you linked to an external stylesheet with this in the <head></head> section:
There is a bit more to CSS than just this but that is the brief intro.
<link rel="stylesheet" type="text/css" src="styles/stylesheet.css" media="screen" />
The good news is server side includes can help do what it sounds like you are after.
Hope it helps.