code: def show_fig_outline_in_viewer(self, state):
          
          if state is True:
              self.figviewer.figcanvas.setStyleSheet(
                      "FigureCanvas{border: 1px solid lightgrey;}")
          else:
              self.figviewer.figcanvas.setStyleSheet("FigureCanvas{}")
          self.option_changed('show_plot_outline', state)
docs: Draw a frame around the figure viewer if state is True.