Gtk3 Transparent Container - css

Question

I'm working on a Gtk3 application. Therefore I designed the window on Ubuntu with Glade 3.16.1.
I have a GtkWindow layout with a vertical seperating GtkPaned. Loading a CSS file I give the window a background image like this:
GtkWindow#window {
background-image: url("Glade Prototype/background.png");
}
But It seems like the layout component is drawn on it with a solid color and so the background image is only partially visible. You can see it
here
My first try was setting the background color of the layout transparent, but it didn't changed anything (Any other color works):
GtkPaned#mainLayout {
background-color: transparent;
/* OR: background-color: rgba(0, 0, 0, 0.0) */
/* OR: background-image: none */
}
Thanks for help,
Greetings Thomas
PS: Does anyone have a Glade version for Windows supporting Gtk3 (+CSS)? I only found Glade 3.8 (CSS supporting versions are e.g. 3.16 / 3.18) on their website.
Edit: Here a better example for this problem:
GtkWindow#window {
background-color: #00FF00;
}
GtkPaned#mainLayout {
background-color: rgba(255, 0, 0, 0.5)
}
-> Image with Alpha 50% / 0.5
You can see that it looks like a mix of red and white color. But i want that white color to disappear.
Edit2:
I've also tried overriding the draw function and drawing a transparent rectangle with Cairo -> No success. My code:
G_MODULE_EXPORT gboolean drawMainContainer(GtkWidget *widget, cairo_t *cr, gpointer data) {
guint width, height;
GdkRGBA color;
width = gtk_widget_get_allocated_width (widget);
height = gtk_widget_get_allocated_height (widget);
cairo_rectangle (cr, 0, 0, width, height);
gtk_style_context_get_color (gtk_widget_get_style_context (widget),
0,
&color);
cairo_set_source_rgba (cr, 0, 0, 0, 0);
cairo_fill (cr);
return FALSE;
}