public void updatePageCounterField(int n) {
   if (pageCounterField == null) return;
   if (n == 0) {
     pageCounterField.setBackground(pageCounterFieldDefaultColor);
     pageCounterField.setForeground(Color.BLACK);
     pageCounterButton.setForeground(pageCounterButtonDefaultForeground);
   } else {
     pageCounterField.setBackground(pageCounterFieldColor);
     pageCounterField.setForeground(Color.WHITE);
     pageCounterButton.setForeground(pageCounterFieldColor);
   }
   pageCounterField.setText("" + n);
 }