The styling is all applied via cascading style sheets (CSS). The
primary CSS is called Core.css. The easiest way to change this would be
to create your own Theme (a collection of graphics and css files) by
copying one of the Theme directories in the 12 hive and making your
changes to the CSS files there. Then apply the theme. A second
possiblility is to add your own custom CSS file to the 12 hive and apply
it to the pages by pointing the AlternateCSSUrl property at your CSS
file. You can either set that property in the ONET.XML <Project>
element of a custom site definition or set the property of the Web
object programmatically. Either of these take a bit more work, but
allow more global management of the CSS file once its done.