how to make the treeitem border invisible

I create the tree and treeitem, then when I run the application, I
found there is a blue border around the treeitem, how to make it
invisible?

in the css file, I already add the following property
.gwt-Tree {
border: 0px;
}

but the border still show!!

the code for building the tree and treeitem is as follow:
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);

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