void gtk_window_set_opacity ( GtkWindow* window, gdouble opacity )
Request the windowing system to make
window partially transparent,
with opacity 0 being fully transparent and 1 fully opaque. (Values
of the opacity parameter are clamped to the [0,1] range.) On X11
this has any effect only on X screens with a compositing manager
running. See gtk_widget_is_composited(). On Windows it should work always.
Note that setting a window’s opacity after the window has been shown causes it to flicker once on Windows.
Use gtk_widget_set_opacity instead.