Well, there are several things to consider before attempting
a "customize for everyone" look, and that is the use of
templates.
1. If you are using a virtual server for each customer, it's
not much problem; you can affect the look of each vs by changing the
css (sps.css for the portal and OWS.css for sites).
2. If you are customizing just the portal and areas, a custom
css can be applied for each new one created – remember, each time
a
new portal, area, or sub-area is created, it will call the
applicable template which will use sps.css by default, requiring you
(or whoever creates it) to go in and apply the custom css.
3. If you want to customize sites it's a much bigger chore,
especially if they are on the same virtual server. Each site, when
created will use OWS.css for the design unless you add a custom site
definition which pulls a different style sheet.
4. Themes have their own css in each of the applicable theme
folders. Each one must be customized individually.
I have customized out portal, but each site underneath uses the same
customization (look and feel) http://public-sp.missouri.edu .
If you change the images, you should not change the name and try to
use an new image that is the same size as the one you are replacing,
otherwise, each template must be touched to apply the new name,
size, etc…. Customizing is not easy because of the architecture
of
SharePoint, but getting rid if the Microsoft blue is attainable.