Re: Automatic face re-application with frame-set-background-mode

> Date: Fri, 21 Aug 2026 08:55:53 +0000
> From: Antonio Romano <[email protected]>
> 
> Il 11 aprile 2026 15:44:30 CEST, Antonio Romano <[email protected]> ha scritto:
> >Hello,
> >
> >I’m experimenting with themes on Emacs (using master branch) and
> >noticed some behavior related to `frame-set-background-mode` that I’d
> >like to clarify. 
> >
> >I created a minimal working example theme as follows:
> >
> >```
> >;; -*- lexical-binding: t; -*-
> >(deftheme mve-theme "Minimal MVE")
> >
> >(custom-theme-set-faces
> > 'default
> > `(default
> >   ((((background light)) :background "white" :foreground "black")
> >    (((background dark))  :background "black" :foreground "white")))
> > `(mode-line
> >   ((((background light)) :background "yellow" :foreground "black")
> >    (((background dark))  :background "red" :foreground "white"))))
> >
> >(provide-theme 'mve-theme)
> >```
> >
> >Steps I performed:
> >
> >Run `emacs -Q`
> >Load the theme with `M-x load-theme mve-theme`.

This step signals an error here:

  custom-theme-set-faces: Unknown theme ‘default’

So I was unable to try reproducing further.

> >Call `(setopt frame-background-mode 'dark)`.
> >
> >Notable observations:
> >
> >* The fringe (unspecified in this theme) changes to black.

Which is okay, right?  Because its defface says:

    (((class color) (background dark))
     :background "grey10")

> >* The mode-line stays yellow.

Not here, it doesn't: it becomes this:

    (((class color grayscale) (min-colors 88) (background dark))
     :box (:line-width -1 :style released-button)
     :background "grey20" :foreground "white")

> >* The default face does not change. This is true even with the
> >`default` theme.

Which is expected, right?

> >* Syntax highlighting colors in emacs-lisp-mode update according to
> >background-mode.

Which is okay, right?

> >It seems that some faces do get re-applied after a `background-mode`
> >switch, while others do not. Even when they do, they do not honor my
> >`custom-theme-set-faces` settings.

No, all of the faces get recalculated.  Are you talking about the
default face only?

> >I’m trying to understand:
> >
> >What conditions determine which faces are automatically recalculated
> >when `frame-set-background-mode` is called?

Basically, all of them are recalculated.

> >Is this behavior hardcoded for built-in faces only?

No.

> >Are there plans or best practices for making theme faces reactive to
> >background-mode changes without manually reapplying the theme?

I don't understand the question, please elaborate.

> At this point I'm pretty sure that `frame-set-backgroud-mode` only recalculates faces for those that are neither user-customized nor themed. I wonder what's the rationale behind that and if it's an intended behaviour. 

The rationale could be that calling frame-set-backgroud-mode at
arbitrary points makes no sense.  The background mode is determined by
the frame's background color, so it should be applied when we first
show a frame, or when the frame's background color changes.