It depends. If you're making site-wide design changes, than it can be as little as modifying a single stylesheet.

However, if you need to make site-wide mark-up changes (add another link, remove a column etc.) then software like Dreamweaver or SSI (Server Side Includes) make the job easier (rather than editing every single page by hand!).

CSS is purely aesthetics, used to modify colours, fonts, layout dimensions, positioning and backgrounds.