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
(among others) which facilitate the reuse of objects such
as page templates amongst a website full of
Next came The Zope
(CMF); it offers technologies like file system directory
views (FSDV) and skin layers (among others). The CMF
and image resources on the filesystem, typically inside a
Initially called Zope 3 (more or less), the
(ZTK) is a set of reusable packages (including
which provide the Zope Component Architecture) that bring
a modern, scalable development environment to Plone.