Although it's not strictly necessary, when icons are added by an extension, running gtk-update-icon-cache will make the search for icons quicker.
The command gtk-update-icon-cache should be run on the directory where the icons are added, for example, /usr/local/share/icons/hicolor.
Following the removal of gtk-update-icon-cache from gtk2/gtk3, some apps will needed the extension gtk-update-icon-cache to be added as a dependency in order to be able to update the icon cache.