To understand Plone's theming story you must first understand the technology stack on which it is built (or not built).
Zope2 is the oldest portion of the stack. It offers technologies like
Acquisition (among others) which facilitate the reuse of objects
such as page templates amongst a website full of "content" objects.
Next came The Zope
Content Management Framework (CMF); it offers
technologies like file system directory views (FSDV) and skin layers (among
image resources on the filesystem, typically inside a "skins" directory.
Initially called Zope 3 (more or less), the
Zope Toolkit (ZTK) is a set of
reusable packages (including
provide the Zope Component Architecture) that bring a modern, scalable
development environment to Plone.