Function: face-remap-add-relative

Add a face remapping entry of FACE to SPECS in the current buffer.
Return a cookie which can be used to delete this remapping with

The remaining arguments, SPECS, should form a list of faces.
Each list element should be either a face name or a property list
of face attribute/value pairs. If more than one face is listed,
that specifies an aggregate face, in the same way as in a `face'
text property, except for possible priority changes noted below.

The face remapping specified by SPECS takes effect alongside the
remappings from other calls to `face-remap-add-relative' for the
same FACE, as well as the normal definition of FACE (at lowest
priority). This function tries to sort multiple remappings for
the same face, so that remappings specifying relative face
attributes are applied after remappings specifying absolute face

The base (lowest priority) remapping may be set to something
other than the normal definition of FACE via `face-remap-set-base'. (fn FACE &rest SPECS)