Hi Malte,
We implemented a similar scheme including having master templates for
http://interlink.azri.biz . Although it is not per user, it will not be a significant challenge to do it that way. Along with it we added a theme upload functionality that took a zipped file which contained the complete directory that a designer would produce. The zip file also consists of a XML manifest file (which contains the mapping between the directory structure and the theme name css folder/images/adp file for the theme) that allows the uploading action to register the theme which is then available to the admin to change. I know there are better ways of doing it but it works quite well for us.
The general directory structure we use is the following:
skin/
master/ - for adp files
www/resources/theme1 - sample theme directory which contains all the image and css files and folders.
I am not sure where to upload this on the openacs site, so in case you are interested I can upload the code somewhere.
