I do a lot of CSS changes to my sites, I find that easier and quicker than to rebuild themes or site definitions and stuff.
MOSS 2007 uses the following css files:
controls.css
HtmlEditorCustomStyles.css
HtmlEditorTableFormats.css
portal.css
core.css
When you create a WSS site, the following CSS files are used:
portal.css
core.css
MySite
portal.css
core.css
