docs: def getInstalledThemes(self, store):
          """
          Collect themes from all offerings installed on this store, or (if called
          multiple times) return the previously collected list.
          """
          if not store in self._getInstalledThemesCache:
              self._getInstalledThemesCache[store] = (self.
                                                   _realGetInstalledThemes(store))
          return self._getInstalledThemesCache[store]