fix wxSystemSettings::GetColour() for wxGTK3

closes #15519

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74839 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/settings.cpp

@@ -38,6 +38,10 @@ static GtkContainer* ContainerWidget()
     {
         s_widget = GTK_CONTAINER(gtk_fixed_new());
         GtkWidget* window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
+#ifdef __WXGTK3__
+        // need this to initialize style for window
+        gtk_widget_get_style_context(GTK_WIDGET(window));
+#endif
         gtk_container_add(GTK_CONTAINER(window), GTK_WIDGET(s_widget));
     }
     return s_widget;