"To make a window transparent, it needs to use a visual which supports that. This is done by getting the RGBA colormap of the screen with gdk_screen_get_rgba_colormap() and setting it on the window. Note that gdk_screen_get_rgba_colormap() will return NULL if transparent windows are not supported on the screen; also note that this may change from screen to screen, so it needs to be repeated whenever the window is moved to a different screen." (с)  http://library.gnome.org/devel/gtk/unstable/gtk-question-index.html