code: public void setSplitterLineColor(Color splitterLineColor) {
          this.splitterLineColor = splitterLineColor;
  
          if (isAttached()) {
              applySplitterLineColor(splitterLineColor);
          } else {
              registerHandler(addAttachHandler(attachEvent -> applySplitterLineColor(splitterLineColor)));
          }
      }
docs: Will set the separator / splitter line color. Default {@link Color#RED}