Virtual Hosting and file structure question

SooperNoob here...

I need to know if what I want to do is (1) possible and (2) how to do it if it is.

I want to set up Apache to do Virtual Hosting on one hardware server, serving
more than 100 web sites. I want to allow the web site clients to have access
to updating their content as far as the text input goes, but I want to have my own
control over CSS, and templates for layout and presentation under my control
through a set of files and scripts that are kept separate from each Virtual Host's
Document Root structure.
How do I go about creating a file structure that can be referenced by the HTML
pages from inside the Virtual Hosts Document Root structure that Apache
serves to requesting browser clients which resides in a file structure not in
the Virtual Hosts Document Root? That is, when the HTML index.html page
is being served from VH Doc. Root, and it makes reference to somescript.php
or some other template file NOT under VH Doc. Root, how can the connection
be made to them? Where do you put it, and what directives do you put into
the Virtual Host container setup in httpd.conf to support that kind of referencing?

The idea here is that I have a lot of supporting files consisting of logic (scripting)
and formatting files (templates, CSS's, etc) that will be common to every
Virtual web host - so extensive that I do not want to fill up valuable disk space
replicating it in each and every Virtual Host's Doc. Root structure. So I want to
break out the common stuff and have it reside physically on disk only once and
be able to refer to it from within the scope of every Virtual server.
Essentially, I'm wanting to know if there is a way to
serve files outside of the DocumentRoot of each Virtual
Host. If so, how does it work.

I've probably been as clear as mud in stating my quest, but that's about as
good as I can make it now with my totally newbie and incomplete understanding
of Apache at this point.

Looking for lots of kind, patient help.

Doug
drobson@chartertn.net