Created attachment 180663 [details]
screenshot

And the rest of the widgets seem to get their bg colour from somewhere else.

I applied this tiny patch to the gtk.css:

@@ -178,7 +178,7 @@ GtkScale {
 
 .notebook {
     -adwaita-selected-tab-color: #8dc0f3;
-    background-color: shade (@bg_color, 1.12);
+    background-color: #ff0000;
     padding: 2;
     border-style: solid;
     border-radius: 0;

And this is the resulting screenshot in gnome-control-center's background panel.

Note that the same problem would happen in glade. Just create 2 separate GtkDrawingArea, one inside a notebook, one outside it.