face-read-string

face-read-string is a compiled Lisp function in `faces.el'.

(face-read-string FACE DEFAULT NAME &optional COMPLETION-ALIST)

Interactively read a face attribute string value.
FACE is the face whose attribute is read. If non-nil, DEFAULT is the
default string to return if no new value is entered. NAME is a
descriptive name of the attribute for prompting. COMPLETION-ALIST is an
alist of valid values, if non-nil.

Entering nothing accepts the default string DEFAULT.
Value is the new attribute value.