Mark,

the absolute Url is /Portals/[PortalID]/portal.css - replace [PortalID] by the actual number. If you don't know the PortalID, go to Host :: SQL and run the following script:

SELECT PortalID FROM {databaseOwner}{objectQualifier}PortalLocalization
WHERE PortalName = '[PortalName]'

Replace [PortalName] by the name of your portal, which you can find under Admin :: Site Settings ("Title").

If you need that in a module code you can use something like:

string pathToPortalCss = string.Format("{0}portal.css", PortalSettings.HomeDirectory);

Happy DNNing!
Michael