Come In And Find Out!

Did you ever wonder how Emacs finds out whether you are using a dark or light background? If you’ve themed it, it can just ask for the current definition of the default face, but if you didn’t do this yet, only GUI Emacs can answer this question truthfully. So, what does Emacs do for the case of an uncustomized setup in a terminal? Simple, it has a hardcoded list of terminal emulators that use light backgrounds by default and does otherwise assume an unknown one uses a dark background! Joke’s on you if you’re using any of these with a dark background and wonder why it’s looking so different…

(let* ((frame-default-bg-mode (frame-terminal-default-bg-mode frame))
       (bg-color (frame-parameter frame 'background-color))
       (tty-type (tty-type frame))
       (default-bg-mode
         (if (or (window-system frame)
                 (and tty-type
                      (string-match "^\\(xterm\\|\\rxvt\\|dtterm\\|eterm\\)"
                                    tty-type)))
             'light
           'dark))
       (non-default-bg-mode (if (eq default-bg-mode 'light) 'dark 'light))
       (bg-mode
        (cond (frame-default-bg-mode)
              ((equal bg-color "unspecified-fg") ; inverted colors
               non-default-bg-mode)
              ((not (color-values bg-color frame))
               default-bg-mode)
              ((>= (apply '+ (color-values bg-color frame))
                   ;; Just looking at the screen, colors whose
                   ;; values add up to .6 of the white total
                   ;; still look dark to me.
                   (* (apply '+ (color-values "white" frame)) .6))
               'light)
              (t 'dark)))
       (display-type
        (cond ((null (window-system frame))
               (if (tty-display-color-p frame) 'color 'mono))
              ((display-color-p frame)
               'color)
              ((x-display-grayscale-p frame)
               'grayscale)
              (t 'mono)))
       (old-bg-mode
        (frame-parameter frame 'background-mode))
       (old-display-type
        (frame-parameter frame 'display-type)))
  ...)

Update: There is no need to customize the default face or patch frame-default-bg-mode, it’s possible to set frame-background-mode to either dark or light instead, similiar to Vim’s background option. Judging by their docs they use heuristics as well, but almost always default to a light background…