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

Antonio Romano <[email protected]> Fri, 21 Aug 2026 08:55:53 +0000

Newsgroups	gmane.emacs.help
Message-ID	<[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`.
>Call `(setopt frame-background-mode 'dark)`.
>
>Notable observations:
>
>* The fringe (unspecified in this theme) changes to black.
>* The mode-line stays yellow.
>* The default face does not change. This is true even with the
>`default` theme.
>* Syntax highlighting colors in emacs-lisp-mode update according to
>background-mode.
>
>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.
>
>I’m trying to understand:
>
>What conditions determine which faces are automatically recalculated
>when `frame-set-background-mode` is called?
>Is this behavior hardcoded for built-in faces only?
>Are there plans or best practices for making theme faces reactive to
>background-mode changes without manually reapplying the theme?
>
>Thank you for any clarification or guidance.

Bumping the question.

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 `background` display entry would be great for defining a light/dark variant in a single theme.

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