sounds like you are a perfect candidate for using the content editor web part to
override the style on the sites  that need changing. check out Todd's blog for a
bunch of examples of how to do different things with the CEWP. Basically,
inserting a CEWP into a page and then adding some CSS you can override the
default CSS classes used by SharePoint on a page by page basis. the beauty of
this, as Todd describes, is that you don't duplicate pages through "unghosting",
and you can revert back to the original style simply by deleting the CEWP.

http://www.mindsharpblogs.com/

You can do a couple things here. If there are already existing sites  you can create a custom theme incorporationg your own css  values or you can do some work with the CEWP and add some css coding there and change  the front facing pages face around.

I have a document on my site  on how to do this http://bobfox.net/spnj/docs.aspx called Quick and Easy Theme Template and I know Mr. Bleeker http://mindsharpblogs/todd has also done some considerable work on this subject.

If this is SPS you're concerned with and you're not concerned with WSS, I would think the easy way to go is to create your own CSS file to override selecte CSS definitions, place it somewhere in the 60 hive and enter the relative path to that CSS file at the bottom of this admin page:

/_layouts/1033/PortalProperties.aspx

This page will give you some ideas of what classes you should use to change  the look of different areas of the page.
msdn.microsoft.com/.../...zingSharePointSites3.asp

But the definitive way to identify the CSS classes and IDs you need to override for the new look, use StyleUnderCursor WebPart.
mindsharpblogs.com/.../801.aspx
Here's another article I always found helpful.
msdn.microsoft.com/.../...plyingcorporatebrand.asp

I’d likely approach this by creating a theme and just setting it on those sites  …