having one CSS file for your entire website is great because you can edit the appearance of your site and all of its content by editing one file. the only real downside of having only one CSS is that all of your pages and content will look the same. if simplicity is what you are looking for, this is the way to go. remember that CSS will always take the most specific order it is given... assigning a separate style to only a certain id or class will have it only apply to that particular part of the page. this can be done in the actual xhtml document, with no need to link to anything externally.
