As Conference Manager, see the "Website Management" link. On Page 4, you can upload a stylesheet to override the built-in styles as defined in styles/common.css. If you like, you can modify common.css, but I'd suggest uploading your own stylesheet so that you don't need to worry about changes to the built-in stylesheet when you upgrade to the next release of OCS.
If you want to make changes that aren't possible using CSS, you'll need to modify the templates. Most of the page layout is done in templates/common/header.css, and although these are Smarty templates, they generally behave like HTML.
Public Knowledge Project Team
Don't miss the First International PKP Scholarly Publishing Conference
July 11 - 13, 2007, Vancouver, BC, Canada