~/.config/gtk-3.0/gtk.css

headerbar {
	margin-top: -100px;
	border-bottom-width: 0;
}

headerbar button,
headerbar spinbutton,
headerbar entry,
headerbar separator {
	margin-top: 103px;
	margin-bottom: 3px;
}

~/.config/gtk-3.0/settings.ini

gtk-decoration-layout=""