lightbox.py

from gi.repository import Gtk, Gdk;

css = """
#MyWindow {
    background-color: rgba(0, 0, 0, 0);
}
"""

overlay = Gtk.Overlay();

def on_button_clicked(source):
    grid = Gtk.Grid(halign = Gtk.Align.CENTER, valign = Gtk.Align.CENTER)
    
    image = Gtk.Image.new_from_file("/usr/share/icons/Adwaita/256x256/apps/help-browser.png")
    win.set_name("MyWindow")

    def hide_grid(source):
        grid.hide()
        win.set_name("NotWindow")

    close_bttn = Gtk.Button(image = Gtk.Image.new_from_icon_name('window-close-symbolic', Gtk.IconSize.MENU))
    close_bttn.connect('clicked', hide_grid)

    grid.attach(image, 0, 1, 1, 1)
    grid.attach(close_bttn, 1, 0, 1, 1)
    overlay.add_overlay(grid)

    grid.show_all()

win = Gtk.Window();
win.connect("delete-event", Gtk.main_quit)
win.maximize();

style_provider = Gtk.CssProvider()
style_provider.load_from_data(css)
Gtk.StyleContext.add_provider_for_screen(Gdk.Screen.get_default(), style_provider, Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION)

win.add(overlay);

box = Gtk.Box(orientation = Gtk.Orientation.VERTICAL)
overlay.add(box)

button = Gtk.Button("show overlay");
button.connect("clicked", on_button_clicked)
box.pack_start(button, False, False, 0)

win.show_all();
Gtk.main();