GtkSettings: Fix theme not updating for dark theme

The theme was not updated when the "gtk-application-prefer-dark-theme"
GtkSettings property changed.

https://bugzilla.gnome.org/show_bug.cgi?id=640983

gtk/gtksettings.c

@@ -1608,6 +1608,7 @@ gtk_settings_notify (GObject    *object,
       settings_update_color_scheme (settings);
       gtk_style_context_reset_widgets (priv->screen);
       break;
+    case PROP_APPLICATION_PREFER_DARK_THEME:
     case PROP_THEME_NAME:
       settings_update_theme (settings);
       break;