Return the background color name of FACE, or nil if unspecified.
If the optional argument FRAME is given, report on face FACE in that frame.
If FRAME is t, report on the defaults for face FACE (for new frames).
If FRAME is omitted or nil, use the selected frame.
If INHERIT is nil, only a background color directly defined by FACE is
considered, so the return value may be nil.
If INHERIT is t, and FACE doesn't define a background color, then any
background color that FACE inherits through its `:inherit' attribute
is considered as well; however the return value may still be nil.
If INHERIT is a face or a list of faces, then it is used to try to
resolve an unspecified background color.
To ensure that a valid color is always returned, use a value of
`default' for INHERIT; this will resolve any unspecified values by
merging with the `default' face (which is always completely specified).