code: def update_highlights(self, old_highlight_set, new_highlight_set):
          
          with self.thmblock:
              un_hilite_set = old_highlight_set - new_highlight_set
              re_hilite_set = new_highlight_set - old_highlight_set
  
              # highlight new labels that should be
              bg = self.settings.get('label_bg_color', 'lightgreen')
              fg = self.settings.get('label_font_color', 'black')
  
              # unhighlight thumb labels that should NOT be highlighted any more
              for thumbkey in un_hilite_set:
                  if thumbkey in self.thumb_dict:
                      namelbl = self.thumb_dict[thumbkey].get('namelbl')
                      namelbl.color = fg
  
              for thumbkey in re_hilite_set:
                  if thumbkey in self.thumb_dict:
                      namelbl = self.thumb_dict[thumbkey].get('namelbl')
                      namelbl.color = bg
  
          if self.gui_up:
              self.c_view.redraw(whence=3)
docs: Unhighlight the thumbnails represented by `old_highlight_set`
          and highlight the ones represented by new_highlight_set.
  
          Both are sets of thumbkeys.