In order to fully understand how themes work, we need to tackle one last
aspect of their conception: the case of implicit faces. Although
Clon itself needs a completely defined face tree to perform output
correctly, a theme file is not required to define all of them (that is
impossible by the way: because of group nesting, a complete theme file
would be infinitely big). In fact, a theme file can be totally empty, in
which case the output will effectively conform to the built-in
‘raw’ theme. When a face is “missing” from a theme, Clon
arranges to define it in a sensible way. Such a face is said to be
implicit. The exact rules for defining implicit faces is what
this section is all about.