Quite often when I design a website for a customer, I design his website with one (or multiple) CSS files that makes up the entire presentation layer. The thing is, usually, the
In situations where a theme is required I, personally, tend to use three base-stylesheets:
There is an awful lot of repetition in this approach, though, so @treefrog's answer may well be a better approach. The one saving grace I can offer for my approach, which is why it works well for me, is that it's easy to know where to go to change the title font from Arial to Times New Roman (or whatever), and where to find the background-colours for the page. Typically these are stored in a Wordpress-like arrangement:
http://www.example.com/css/reset.css
http://www.example.com/css/themeName/typography.css
http://www.example.com/css/themeName/layout.css