You are misunderstood the purpose of CSS/CSS2.
The purpose of CSS is to centralize the website style.
The purpose of CSS2 (CSS-P) is to positioning webpage element.
Of course, CSS2 spec haven't reach the sastified state for designer, so CSS3 spec is being developed.
To seperate data/layout you should know what is the layout and what is the data.
The data, you have already know I'm sure. But the layout here are both (X)HTML code and CSS file.
If you thinks that data are (X)HTML files, are your header, footer your data? They are your layout element, to change the layout, you have to redesign it.
About the *outline* of your data, I won't think you will use CSS-P to position it... if you do, you're a kind of odds people ^_^
Take a look, this is my next template in development. I'm newbie about CSS (start to study it for 2 weeks I think) so don't expect too much from it. But I think you can learn something from it like me when I learn to design... ^_^