I think you are confusing an oacs subsite with a physical folder in your file system. As far as I know you cannot have an own "package-directory" for each subsite. You can just mount packages in it and have some administrative possibilities which are only valid to these packages.
If you want personalized layout you should create a master template file that looks from what package and subsite it is called and then applies the appropriate css-file,...
https://openacs.org/doc/acs-subsite/
https://openacs.org/api-doc/package-view?version_id=358370