How to change to dark theme of an application with LGI (Lua + GTK)

ToggleDarkMode.lua

-- I require LGI
local lgi = require("lgi")
local Gtk = lgi.require("Gtk", "3.0")

-- I create the application
local app = Gtk.Application {
	-- The application ID
	application_id = "com.gists.github.diazvictor.ToggleDarkMode"
}

function app:on_startup()
	-- I create a configuration
	settings = Gtk.Settings.get_default()
	settings.gtk_application_prefer_dark_theme = false

	-- Creates the window
	Gtk.ApplicationWindow {
		application = self,
		window_position = Gtk.WindowPosition.CENTER,
		default_width = 600,
		default_height = 400
	}
end

function app:on_activate()
	-- The window
	local window = self.active_window 
	-- I create a header
	local headerbar = Gtk.HeaderBar {
		-- to display a widget
		visible = true,
		-- I show the buttons of the window (close, minimize and maximize).
		show_close_button = true,
		-- Here is the title of the window
		title = "Toggle dark variant",
		{
			-- I create a button with two states (active and inactive)
			Gtk.ToggleButton {
				visible = true,
				-- I add an image to know the current status
				Gtk.Image {
					id = 'state',
					visible = true,
					-- I set an icon
					icon_name = 'weather-clear-night-symbolic'
				},
				on_toggled = function (self)
					-- This is how I access the GtkImage
					local state = self:get_parent().child.state
					if self.active then
						-- The icon for light mode
						state.icon_name = 'weather-clear-symbolic'
					else
						-- The icon for dark mode
						state.icon_name = 'weather-clear-night-symbolic'
					end
					-- Change to dark theme depending on button status (true or false)
					settings.gtk_application_prefer_dark_theme = self.active
				end
			},
			-- I place the button at the end
			pack_type = 'END'
		}
	}
	-- I add to the window the header
	window:set_titlebar(headerbar)
	--- This is just to fill in the window void.
	-- I create a box
	local box = Gtk.Box {
		visible = true,
		-- Change its orientation to vertical
		orientation = Gtk.Orientation.VERTICAL,
		-- I align it to the center both vertically and horizontally.
		halign = Gtk.Align.CENTER,
		valign = Gtk.Align.CENTER,
		-- I add a label
		Gtk.Label {
			visible = true,
			-- I add a text
			label = 'Press the button above'
		}
	}

	-- I add the box to the window
	window:add(box)
	window:present()
end

-- I run the application
app:run()