gtk-demo: Dark theme support for dnd demo

Add minimal dark theme support to the dnd demo,
by picking suitable default colors for our items.

demos/gtk-demo/dnd.c

@@ -153,6 +153,30 @@ click_done (GtkGesture *gesture)
     gtk_widget_insert_after (item, canvas, last_child);
 }
 
+static gboolean
+theme_is_dark (void)
+{
+  GtkSettings *settings;
+  char *theme;
+  gboolean prefer_dark;
+  gboolean dark;
+
+  settings = gtk_settings_get_default ();
+  g_object_get (settings,
+                "gtk-theme-name", &theme,
+                "gtk-application-prefer-dark-theme", &prefer_dark,
+                NULL);
+
+  if ((strcmp (theme, "Adwaita") == 0 && prefer_dark) || strcmp (theme, "HighContrastInverse") == 0)
+    dark = TRUE;
+  else
+    dark = FALSE;
+
+  g_free (theme);
+
+  return dark;
+}
+
 static void
 canvas_item_init (CanvasItem *item)
 {
@@ -180,7 +204,11 @@ canvas_item_init (CanvasItem *item)
   gtk_widget_set_name (item->label, id);
   g_free (id);
 
+  if (theme_is_dark ())
+    gdk_rgba_parse (&rgba, "blue");
+  else
     gdk_rgba_parse (&rgba, "yellow");
+
   set_color (item, &rgba);
 
   item->angle = 0;