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

> From: Antonio Romano <[email protected]>
> Cc: [email protected]
> Date: Fri, 21 Aug 2026 14:35:52 +0000
> 
> Eli Zaretskii <[email protected]> writes:
> > This step signals an error here:
> >
> >   custom-theme-set-faces: Unknown theme ‘default’
> >
> > So I was unable to try reproducing further.
> 
> 
> That totally slipped my eye. Sorry. Here's a version that works on my
> side:

That still signals an error about an unknown theme 'mve'.

> > 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")
> 
> If `mve` theme is applied, then it does stay yellow. Tell me if you
> concur with this behaviour. Unlike the uncostumized face discussed right
> above (fringe), this face does not get recalculated to its dark
> spec. This is inconsistent and makes me doubt about its intended usage.

I think the intended usage is that the background mode is set _before_
the theme's faces are applied.

> >> >* The default face does not change. This is true even with the
> >> >`default` theme.
> > Which is expected, right?
> 
> This makes the less sense. With `emacs -Q`, setting
> `frame-background-mode` to 'dark then you'll notice that there are
> recomputed faces for dark background but the `default` background stays
> the light one. If you open C-x C-f you'll se cyan text on white
> background.

Again, you have it backwards: AFAIU, the background mode should be
known _before_ the default faces are determined.

> >> 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.
> 
> `frame-background-mode` is a user option, thus it seems to me that it
> should be called at arbitrary points. I thought it was the source of
> truth for determining the background color.

It's the other way around, actually: the background color is the
source for determining the background mode.

> >> >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.
> 
> Implying that `frame-background-mode` is a user option (which is
> debatable), how should one make a theme that accounts for such
> specification? With what I've seen above, this acts in inconsistent
> ways, or maybe I'm just abusing or misusing it.

Load the theme after the background mode is set.  Does that behave as
expected?