Forum OpenACS Q&A: Re: Current state of theming in OpenACS (without .LRN)
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.
I hope this is of some help.