How do I duplicate the 'effective' theme colors resulting from using terminal Emacs with a transparent background?

Question

I'm running GNU Emacs 24.5.4 on Mac OS X 10.10.5 in Terminal.app.

In this (stock) setup, Emacs has a transparent background. With a transparent background, the colors that Emacs uses to display text are auto-adjusted based on the color underneath Emacs. In my case, "underneath Emacs" is a Terminal.app window with an opaque black background.

(In the title I refer to the "effective" theme. I'm using that to mean to the colors that Emacs actually uses to render text - as opposed to the "specified" theme, which is different in this case.)

Here is what the default theme looks like when no background color is specified in Emacs, and Emacs is running in a Terminal.app window with an opaque black background:

And here is the same setup - except with a black background specified in Emacs:

While beauty is certainly in the eye of the beholder, I've gotten used to the colors in the top image.

So I'd like to duplicate those colors in GUI Emacs. But since GUI Emacs isn't a transparent window always running on top of something with a black background and hence having colors auto-adjusted, setting its background to black results in the coloring shown in the bottom screenshot.

How do I duplicate the 'effective' colors used in the top screenshot so that they are used in GUI Emacs as well?

P.S. Here's a video demonstrating the text color adjustments happening in real-time vs. changes to the background color of Terminal.app: vimeo.com/139967912. (I had to use an external camera since QuickTime Player's screen recording didn't work for this purpose.)

Answer 1

This isn't what you think it is. Emacs has hilariously terrible heuristics to determine whether your background in an uncustomized terminal session is dark or light. What you see here is that it has detected your terminal emulator to come with a light background and is therefore using the light faces on a dark background. In a GUI session the heuristics won't fail, so it's showing the dark faces.

As for how to achieve this, feel free to patch frame-set-background-mode or create your own theme that mirrors the aesthetics of this visual glitch...

Answer 2

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)))
  ...)

From http://emacshorrors.com/posts/come-in-and-find-out.html. Licensed Creative Commons Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0).

Answer 3

Modifying frame-set-background-mode (as suggested in @wasamasa's answer) isn't necessary.

You only need to override Emacs' detection of the frame color. In my case, since I like the color scheme when Emacs thinks I have a light-colored frame but I actually have a dark-colored frame, all I need to do is

(setq frame-background-mode 'light)

More here: http://www.gnu.org/software/emacs/manual/html_node/emacs/Faces.html

Tricking Emacs in this way can lead to bad effects: black-on-black text for example.