Mark,
My understanding is that you are correct. You would create a new package, use whatever already exists that is suitable as a boilerplate, and then modify to suit.
I believe that the parameters you are referring to enable you to reference your own CSS files on a per-subsite basis, without you having to make a custom package.
I also have not seen a thing specific known as a 'theming UI'. It could be referring to the user interface of the layout-manager package (and the layout-managed-subsite package which is based on it). The admin UI allows you to define the format of your pages and to place various 'includelets' as you want.
One other thing to mention, have you seen or used Xowiki? This is in fact such a flexible and powerful interface to the OpenACS content repository that there is unlikely to be a need to code anything. Certainly creating data/content entry forms paired to templates that can be fully customised with CSS and Javascript includes is possible without any coding or OpenACS customisation at all. Fragments of content can also be brought forward into pages from all over the place with very little effort.
I hope this is of some help.
Regards
Richard
