As documented in Understanding the style.config file
Css files that live in the skin folder are added very simply with an entry like this for each file:
the above syntax only works for files in the skin folder of the current skin you can't put a path to other folders there.
Create a folder in a common location like at /Data/mystyles or /Data/Sites/1/mystyles and put the shafred css files there
then use syntax like this:
<file cssvpath="/Data/mystyles/style.css" imagebasevpath="/Data/mystyles/">none</file>
don't store the common css in a skin folder but in a central location outside the skin folders
Hope that helps,