Basically you are copying a package so the .info file needs to reflect the package key for your new package.
I do not inherit templates for the theme package. The subsite derived package needs to inherit templates and needs to "extend" acs-subsite. This way you can put only the files that are different under your derived subsite package and use the default subsite templates where appropriate.
Since a theme package is just a master template and associated css, list and form templates, inheriting doesn't make sense for the theme.
