In this case, a frame and skin arrangement. The frame, which we'll call Art Class, handles all the mechanical stuff. The skins, which we'll call Projects, handle appearance. So Art Class would handle things like threaded comments, while each Art Class Project would handle how those threaded comments appear on the screen. So a Snow Day Art Class Project would present threaded comments in a winter motif.
Art Class itself would have no style sheet, no CSS at all. All presentation would be handled by the skin, by the Project. Each Project would be handled solely and entirely by a style sheet. The base Art Class installation would come with two or three Projects, which the user could switch between as he desires. As an option one could give readers of the blog to switch between Projects as they wish.
In addition Art Class would come with with an options page whereby the user could set the number of sidebars, the type of header and footer, and where everything is located. For instance one could have three left hand sidebars, one top for author information and calender, and two side by side beneath it for other stuff. How those sidebars look being set by the Project or style sheet.
So keep this in mind; Art Class is the theme engine, while each Art Class Project is the decoration. Neither is a complete theme in and of itself, the theme is Art Class and Project together, and different Projects can use the same Art Class. I hope this made sense.
Have fun with it.