Function: read-face-name

Read one or more face names, defaulting to the face(s) at point.
PROMPT should be a prompt string; it should not end in a space or
a colon.

The optional argument DEFAULT specifies the default face name(s)
to return if the user just types RET. If its value is non-nil,
it should be a list of face names (symbols); in that case, the
default return value is the `car' of DEFAULT (if the argument
MULTIPLE is non-nil), or DEFAULT (if MULTIPLE is nil). See below
for the meaning of MULTIPLE.

If DEFAULT is nil, the list of default face names is taken from
the `read-face-name' property of the text at point, or, if that
is nil, from the `face' property of the text at point.

This function uses `completing-read-multiple' with "," as the
separator character. Thus, the user may enter multiple face
names, separated by commas. The optional argument MULTIPLE
specifies the form of the return value. If MULTIPLE is non-nil,
return a list of face names; if the user entered just one face
name, the return value would be a list of one face name.
Otherwise, return a single face name; if the user entered more than one face name, return only the first one.