This may not answer your question... I'll try to make it as simple as possible I suppose. The whole point of CSS is just a way of making sure all of your pages are the same as your other pages (pretty much templates). If you understand how to set up a page in HTML using styles/attributes, you know CSS.
Example HTML using divs...
<div id="content" style="float:left; width:975px; height:394px; text-align:center; margin:0 auto;">Stuff</div>
Example of how you would do it in separate CSS file...
div#content
{
Float: Left;
width:975px;
height:394px;
text-align:center;
margin:0 auto;
}
by doing it with CSS all you have to do is make the div ID=content (in this case) anywhere you want the same formatting. There is rarely a need to make multiple style sheets (unless its special circumstances). I usually write out the styles in html then remove them and make a CSS file from there (to make sure it all works first).