cpp-face-default-list is a variable defined in `cpp.el'.
Its value is
(("default" . default) ("invisible" . invisible))

Original value was

Alist of faces you can choose from for cpp conditionals.
Each element has the form (STRING . FACE), where STRING
serves as a name (for `cpp-highlight-buffer' only)
and FACE is either a face (a symbol)
or a cons cell (background-color . COLOR).

You can customize this variable.