<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_icon_name(self, icon_id: str) -> str: """Lookup the system icon name from udisie-internal id."""

        icon_theme = Gtk.IconTheme.get_default()
        for name in self._icon_names[icon_id]:
            if icon_theme.has_icon(name):
                return name
        return 'not-available'