Advice needed - handling configuration files

Hi all,

Git newbie here, so please spell things out.  I've been reading the docs 
and got quite confused  :)

I have a web application that has been developed for one customer. It 
is  just about to go to three customers -each will need their own config 
file, css file and directory of images. Each customer may have a test 
and production version that differ only in the config file. The 
developer (me) also needs configuration files for various purposes 
(scripted testing and manual testing).

What is the best way of handling this situation, so that the config, css 
and image files are under version control, without having to repeat 
changes in  three branches?

If a patch set could be created that contained all but the changes to 
the configuration files, then that could be pulled up into the 
mothership, and each satellite can have its own configuration. I want to 
avoid renaming/copying files if possible because that is change that 
goes into production untested.

Regards

Ian