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

[Maximiliano Sandoval]

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.

Note that there are already some hacks to set the header bar to dark mode, but they depend on X11isms, see https://github.com/bepvte/gtk-variant.el.

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Maximiliano