Face Remapping

The variable face-remapping-alist is used for buffer-local or global changes in the appearance of a face. For instance, it is used to implement the text-scale-adjust command (see Text Scale).

— Variable: face-remapping-alist

The value of this variable is an alist whose elements have the form (face . remapping). This causes Emacs to display any text having the face face with remapping, rather than the ordinary definition of face.
remapping may be any face spec suitable for a face text property: either a face (i.e., a face name or a property list of attribute/value pairs), or a list of faces. For details, see the description of the face text property in Special Properties. remapping serves as the complete specification for the remapped face—it replaces the normal definition of face, instead of modifying it.
If face-remapping-alist is buffer-local, its local value takes effect only within that buffer.
Note: face remapping is non-recursive. If remapping references the same face name face, either directly or via the :inherit attribute of some other face in remapping, that reference uses the normal definition of face. For instance, if the mode-line face is remapped using this entry in face-remapping-alist:
          (mode-line italic mode-line)
then the new definition of the mode-line face inherits from the italic face, and the normal (non-remapped) definition of mode-line face.

The following functions implement a higher-level interface to face-remapping-alist. Most Lisp code should use these functions instead of setting face-remapping-alist directly, to avoid trampling on remappings applied elsewhere. These functions are intended for buffer-local remappings, so they all make face-remapping-alist buffer-local as a side-effect. They manage face-remapping-alist entries of the form

where, as explained above, each of the relative-spec-N and base-spec is either a face name, or a property list of attribute/value pairs. Each of the relative remapping entries, relative-spec-N, is managed by the face-remap-add-relative and face-remap-remove-relative functions; these are intended for simple modifications like changing the text size. The base remapping entry, base-spec, has the lowest priority and is managed by the face-remap-set-base and face-remap-reset-base functions; it is intended for major modes to remap faces in the buffers they control.

— Function: face-remap-add-relative face &rest specs

This function adds the face spec in specs as relative remappings for face face in the current buffer. The remaining arguments, specs, should form either a list of face names, or a property list of attribute/value pairs.
The return value is a Lisp object that serves as a cookie; you can pass this object as an argument to face-remap-remove-relative if you need to remove the remapping later.
          ;; Remap the 'escape-glyph' face into a combination
          ;; of the 'highlight' and 'italic' faces:
          (face-remap-add-relative 'escape-glyph 'highlight 'italic)
          
          ;; Increase the size of the 'default' face by 50%:
          (face-remap-add-relative 'default :height 1.5)

— Function: face-remap-remove-relative cookie

This function removes a relative remapping previously added by face-remap-add-relative. cookie should be the Lisp object returned by face-remap-add-relative when the remapping was added.

— Function: face-remap-set-base face &rest specs

This function sets the base remapping of face in the current buffer to specs. If specs is empty, the default base remapping is restored, similar to calling face-remap-reset-base (see below); note that this is different from specs containing a single value nil, which has the opposite result (the global definition of face is ignored).
This overwrites the default base-spec, which inherits the global face definition, so it is up to the caller to add such inheritance if so desired.

39.12.5 Face Remapping