code: public void addFill() {
          middle = FILL;
          cssValue = cssValue.replace(FILL, "").concat(" ").concat(FILL);
          if (getStateChangeInformer() != null) {
              getStateChangeInformer().stateChanged(this);
          }
      }
docs: adds the value {@code fill} in cssValue.
  
  @since 1.0.0
  @author WFF