The majority of Plone's theming machinery is built on Zope 2 and exposed though the ZMI in a tool called
portal_skins, at the root of every Plone site.
portal_skins are folders known as skin layers, additional places Plone will look for things. When a theme resource is needed for example, when an image is requested from the URL http://example.com/plonesite/folder1/folder2/image.jpg Zope looks for it in the following places in a predictable order, following a scheme called acquisition:
image.jpg, just as the address says. If it is not found here, Zope continues on...