buffer-face-mode-face is a variable defined in `face-remap.el'.
Its value is

The face specification used by `buffer-face-mode'.
It may contain any value suitable for a `face' text property,
including a face name, a list of face names, a face-attribute
plist, etc.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.