docs: def _add_decoration(self, cursor):
          """
          Adds a decoration for the word under ``cursor``.
          """
          if self._deco is None:
              if cursor.selectedText():
                  self._deco = TextDecoration(cursor)
                  if self.editor.background.lightness() < 128:
                      self._deco.set_foreground(QtGui.QColor('#0681e0'))
                  else:
                      self._deco.set_foreground(QtCore.Qt.blue)
                  self._deco.set_as_underlined()
                  self.editor.decorations.append(self._deco)
                  self.editor.set_mouse_cursor(QtCore.Qt.PointingHandCursor)
              else:
                  self.editor.set_mouse_cursor(QtCore.Qt.IBeamCursor)