GtkIconFactory::add
void add(GtkIconSet icon_set, string stock_id);
This method adds a specified icon_set to the icon factory under the name stock_id. The stock id you specify should include the name of your application. Normally applications create a GtkIconFactory and add it to the list of default factories with add_default() . Then they pass the stock id to widgets like GtkImage to display the icon.
Themes can provide an icon with the same name to override your application's default icons. If an icon already existed in the icon factory for stock_id, it is unreferenced and replaced with the new icon_set.