So it's more like:
-- UIX (source files - disabled from view, do not edit directly)
----- BASE Theme (make all major edits here that you want applied to both, disabled from view though)
---------- UI.X Dark (source files, uploaded manually from child XML - disabled from view, do not edit directly)
--------------- DARK - the theme that is enabled for viewing (copy of child XML for editing purposes)
---------- LIGHT - the theme that is enabled for viewing
I'll have to update Dark manually each time and won't be able to use the UI.X addon, correct?