The concept of a cascading style sheet, is to change the look and feel of an entire web site, by making changes to a single file propagate an entire site..
Most SharePoint portals I see, doesn’t use a cascading style sheet (css).
The SharePoint technology does have a similar functionality, with the master pages and site themes.

If css are to be used, how and where should they be used?
Firstly, a CSS needs to be designed with the desired look and feel. This could be done by graphic design firm or Interdyn.

Then the design needs to be applied to every page of the site and component of the site, by InterDyn.

Summary

CSS are particularly useful, if a site is being rebranded every couple of years, or there are always additional HTML pages being added, which is not the case with most SharePoint site web sites.

If you feel the site will need to be re branded every couple of years, which is not a bad thing, then a CSS style sheet is required.

However, in any corporate intranet, content is king. This is what brings people back again.