how to make the border of tree widget invisible

[peter tong]

as the image shown, there is a blue border surrounding the tree widget and the menu widget, how to get this blue border invisible?
I already try to set

.gwt-Tree {
	border: 0px;
}

but the tree still shown the blue border.

the complete code is as follow:

 * Copyright 2011 Google Inc. All Rights Reserved.
package com.mycompany.project.client;

import com.google.gwt.core.client.EntryPoint;

/**
 * Entry point classes define <code>onModuleLoad()</code>.
 */
public class GWTDesignerFirst implements EntryPoint {
	public void onModuleLoad() {
		RootPanel rootPanel = RootPanel.get();
		
		final HorizontalSplitPanel horizontalSplitPanel = new HorizontalSplitPanel();
		rootPanel.add(horizontalSplitPanel, 10, 10);
		horizontalSplitPanel.setSize("414px", "264px");
		
		TabPanel tabPanel = new TabPanel();
		horizontalSplitPanel.setLeftWidget(tabPanel);
		tabPanel.setSize("100%", "100%");
		final FormPanel formPanel = new FormPanel();
		FlowPanel flowPanel1 = new FlowPanel();
		formPanel.add(flowPanel1);
		horizontalSplitPanel.setRightWidget(formPanel);
		formPanel.setSize("100%", "100%");
		
		Tree tree = new Tree();		
		tree.addTreeListener(new TreeListener(){

			@Override
			public void onTreeItemSelected(TreeItem item) {
				// TODO Auto-generated method stub
				if (item.getHTML().equalsIgnoreCase("menu1")){
					horizontalSplitPanel.setRightWidget(new FlowPanel());
				}else{
					horizontalSplitPanel.setRightWidget(formPanel);
				}
			}

			@Override
			public void onTreeItemStateChanged(TreeItem item) {
				// TODO Auto-generated method stub
				
			}
			
		});
		tabPanel.add(tree, "New tab", false);
		//tree.setSize("5cm", "3cm");
		
		TreeItem trtmNewItem = new TreeItem("menu1");
		tree.addItem(trtmNewItem);
		trtmNewItem.setState(true);
		
		TreeItem trtmMenu = new TreeItem("menu2");
		tree.addItem(trtmMenu);
		
		TreeItem trtmSubmenu = new TreeItem("submenu2");
		trtmMenu.addItem(trtmSubmenu);
		trtmMenu.setState(true);
		
		
		HorizontalPanel horizontalPanel1 = new HorizontalPanel();
		flowPanel1.add(horizontalPanel1);
		Label lbl1 = new Label("user name");		
		TextBox txtbox1 = new TextBox();
		horizontalPanel1.add(lbl1);
		horizontalPanel1.add(txtbox1);
		txtbox1.setWidth("114px");
		flowPanel1.add(horizontalPanel1);		
		Button btnNewButton = new Button("New button");
		flowPanel1.add(btnNewButton);
		RootPanel.get("main").add(horizontalSplitPanel);
		
		
		
	}
}