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39.12.2 Defining Faces

The usual way to define a face is through the defface macro. This macro associates a face name (a symbol) with a default face spec. A face spec is a construct which specifies what attributes a face should have on any given terminal; for example, a face spec might specify one foreground color on high-color terminals, and a different foreground color on low-color terminals.

People are sometimes tempted to create a variable whose value is a face name. In the vast majority of cases, this is not necessary; the usual procedure is to define a face with defface, and then use its name directly.

Note that once you have defined a face (usually with defface), you cannot later undefine this face safely, except by restarting Emacs.

— Macro: defface face spec doc [keyword value]...

This macro declares face as a named face whose default face spec is given by spec. You should not quote the symbol face, and it should not end in ‘-face’ (that would be redundant). The argument doc is a documentation string for the face. The additional keyword arguments have the same meanings as in defgroup and defcustom (see Common Keywords).

If face already has a default face spec, this macro does nothing.

The default face spec determines face's appearance when no customizations are in effect (see Customization). If face has already been customized (via Custom themes or via customizations read from the init file), its appearance is determined by the custom face spec(s), which override the default face spec spec. However, if the customizations are subsequently removed, the appearance of face will again be determined by its default face spec.

As an exception, if you evaluate a defface form with C-M-x in Emacs Lisp mode (eval-defun), a special feature of eval-defun overrides any custom face specs on the face, causing the face to reflect exactly what the defface says.

The spec argument is a face spec, which states how the face should appear on different kinds of terminals. It should be an alist whose elements each have the form 

          (display . plist)

display specifies a class of terminals (see below). plist is a property list of face attributes and their values, specifying how the face appears on such terminals. For backward compatibility, you can also write an element as (display plist).

The display part of an element of spec determines which terminals the element matches. If more than one element of spec matches a given terminal, the first element that matches is the one used for that terminal. There are three possibilities for display:

default
This element of spec doesn't match any terminal; instead, it specifies defaults that apply to all terminals. This element, if used, must be the first element of spec. Each of the following elements can override any or all of these defaults.
t
This element of spec matches all terminals. Therefore, any subsequent elements of spec are never used. Normally t is used in the last (or only) element of spec.
a list
If display is a list, each element should have the form (characteristic value...). Here characteristic specifies a way of classifying terminals, and the values are possible classifications which display should apply to. Here are the possible values of characteristic:
type
The kind of window system the terminal uses—either graphic (any graphics-capable display), x, pc (for the MS-DOS console), w32 (for MS Windows 9X/NT/2K/XP), or tty (a non-graphics-capable display). See window-system.
class
What kinds of colors the terminal supports—either color, grayscale, or mono.
background
The kind of background—either light or dark.
min-colors
An integer that represents the minimum number of colors the terminal should support. This matches a terminal if its display-color-cells value is at least the specified integer.
supports
Whether or not the terminal can display the face attributes given in value... (see Face Attributes). See Display Face Attribute Testing, for more information on exactly how this testing is done.

If an element of display specifies more than one value for a given characteristic, any of those values is acceptable. If display has more than one element, each element should specify a different characteristic; then each characteristic of the terminal must match one of the values specified for it in display.

For example, here's the definition of the standard face highlight: 

     (defface highlight
       '((((class color) (min-colors 88) (background light))
          :background "darkseagreen2")
         (((class color) (min-colors 88) (background dark))
          :background "darkolivegreen")
         (((class color) (min-colors 16) (background light))
          :background "darkseagreen2")
         (((class color) (min-colors 16) (background dark))
          :background "darkolivegreen")
         (((class color) (min-colors 8))
          :background "green" :foreground "black")
         (t :inverse-video t))
       "Basic face for highlighting."
       :group 'basic-faces)

Internally, Emacs stores each face's default spec in its face-defface-spec symbol property (see Symbol Properties). The saved-face property stores any face spec saved by the user using the customization buffer; the customized-face property stores the face spec customized for the current session, but not saved; and the theme-face property stores an alist associating the active customization settings and Custom themes with the face specs for that face. The face's documentation string is stored in the face-documentation property.

Normally, a face is declared just once, using defface, and any further changes to its appearance are applied using the Customize framework (e.g., via the Customize user interface or via the custom-set-faces function; see Applying Customizations), or by face remapping (see Face Remapping). In the rare event that you need to change a face spec directly from Lisp, you can use the face-spec-set function.

— Function: face-spec-set face spec &optional spec-type

This function applies spec as a face spec for face. spec should be a face spec, as described in the above documentation for defface.

This function also defines face as a valid face name if it is not already one, and (re)calculates its attributes on existing frames.

The optional argument spec-type determines which spec to set. If it is omitted or nil or face-override-spec, this function sets the override spec, which overrides face specs on face of all the other types mentioned below. This is useful when calling this function outside of Custom code. If spec-type is customized-face or saved-face, this function sets the customized spec or the saved custom spec, respectively. If it is face-defface-spec, this function sets the default face spec (the same one set by defface). If it is reset, this function clears out all customization specs and override specs from face (in this case, the value of spec is ignored). The effect of any other value of spec-type on the face specs is reserved for internal use, but the function will still define face itself and recalculate its attributes, as described above.