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GNU Emacs Lisp Reference Manual

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38.11.2 Defining Faces

The way to define a new face is with defface. This creates a kind of customization item (see section 14. Writing Customization Definitions) which the user can customize using the Customization buffer (see section `Easy Customization' in The GNU Emacs Manual).

Macro: defface face spec doc [keyword value]...
This declares face as a customizable face that defaults according to spec. You should not quote the symbol face. The argument doc specifies the face documentation. The keywords you can use in defface are the same ones that are meaningful in both defgroup and defcustom (see section 14.1 Common Item Keywords).

When defface executes, it defines the face according to spec, then uses any customizations that were read from the init file (see section 40.1.2 The Init File, `.emacs') to override that specification.

The purpose of spec is to specify how the face should appear on different kinds of terminals. It should be an alist whose elements have the form (display atts). Each element's CAR, display, specifies a class of terminals. The element's second element, atts, is a list of face attributes and their values; it specifies what the face should look like on that kind of terminal. The possible attributes are defined in the value of custom-face-attributes.

The display part of an element of spec determines which frames the element applies to. If more than one element of spec matches a given frame, the first matching element is the only one used for that frame. There are two possibilities for display:

This element of spec matches all frames. 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 frames, and the values are possible classifications which display should apply to. Here are the possible values of characteristic:

The kind of window system the frame uses--either graphic (any graphics-capable display), x, pc (for the MS-DOS console), w32 (for MS Windows 9X/NT), or tty (a non-graphics-capable display).

What kinds of colors the frame supports--either color, grayscale, or mono.

The kind of background--either light or dark.

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 frame must match one of the values specified for it in display.

Here's how the standard face region is defined:

(defface region
  `((((type tty) (class color))
     (:background "blue" :foreground "white"))
    (((type tty) (class mono))
     (:inverse-video t))
    (((class color) (background dark))
     (:background "blue"))
    (((class color) (background light))
     (:background "lightblue"))
    (t (:background "gray")))
  "Basic face for highlighting the region."
  :group 'basic-faces)

Internally, defface uses the symbol property face-defface-spec to record the face attributes specified in defface, saved-face for the attributes saved by the user with the customization buffer, and face-documentation for the documentation string.

User Option: frame-background-mode
This option, if non-nil, specifies the background type to use for interpreting face definitions. If it is dark, then Emacs treats all frames as if they had a dark background, regardless of their actual background colors. If it is light, then Emacs treats all frames as if they had a light background.

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