The beauty of CSS...
More than any other item, the promise of CSS and XHTML is "to separate the content from the presentation". For those of you not familiar with "style sheets", it's simply a method of building a web site whose individual pages contain only the content of the page, and none of it's graphical descriptors. ("font" tags, and other graphic commands for colors, backgrounds, tables and images) Instead of repeating this information on every page, a "style sheet" contains all those elements, and serves as an external reference for your entire site. This means that you can graphically update your entire site, without touching the individual pages, by simply switching to another style sheet. (Click the blue links above.)
The samples above illustrate this point, by applying many different style sheets to the exact same content. Click any of the blue links at the top of this page to view various color schemes.
NOTE: This is only a simple "demo". The samples here illustrate how a style sheet change can affect an entire site, and are not typically "good enough" to be used on an actual modern site. See the Sites I've Designed page, for higher quality work.