bug#53457: 29.0.50; pgtk: [Request] Expose GtkSettings to emacs

[Po Lu]

Maximiliano Sandoval <msandova@protonmail.com> writes:

> Now that Emacs has a proper GTK header bar it would be interesting to
> have some GtkSettings exposed, such as
> gtk-application-prefer-dark-theme for example, which would allow to
> programmatically set dark mode for the headerbar.

We don't have a "proper GTK header bar", only the fallback title bar GTK
uses for window decoration, which is only available on window systems
where server side decorations don't exist (such as Wayland and
Broadway.)

In addition to that, the setting is deprecated, so I think relying on it
to set the stylesheet used for window decorations will not be very
effective.

People will probably start screaming like goats if we enable the
fallback title bar on X as well, since client-side decorations are a
very controversial subject, so the best option would be to utilize the
system-wide settings for that job.

Thanks.