Classic is already as much of a Theme as it can be.
The complete lack of ColorStyles in UIBuilder prevents me from globally updating things like title color, label color, table colors, etc. from the _FetishStyles.xml file. If SOE would implement ColorStyles (I've asked several times) all I'd have to do to support Multiple sets is replace the Fetish images and Styles.
As it stands now, any set that uses different XML defined colors requires its own set of window files. Unfortunately I haven't had the free time required to update another set
