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