Also, if you put your CSS in a separate file then you will save a modicum of bandwith and make happy your users with slow connections. If you use all the same style sheet for each page on your site then browsers will only need to download the file once and non-visual and text-only browsers will not have to download the file at all.
“The power of the Web is in its universality. Access by everyone regardless of disability is an essential aspect.”
—Tim Berners-Lee, W3C Director and inventor of the World Wide Web