Here is a small snippet about how to simulate rounded borders.

var leftImg = "widget/own/buttonbar_left.png";
var centerImg = "widget/own/buttonbar_center.png",
var rightImg = "widget/own/buttonbar_right.png"
 
var bar = new qx.ui.layout.HorizontalBoxLayout;
			
bar.set({
	width : "100%",
	height : 26,
	verticalChildrenAlign : "middle"
});
 
var barLeft = new qx.ui.layout.CanvasLayout;
barLeft.set({
	width: 5,
	height : 26,
	backgroundImage : leftImg
});
 
var barCenter = new qx.ui.layout.CanvasLayout;
barCenter.set({
	width: "1*",
	height : 26,
	verticalChildrenAlign : "middle",
	horizontalChildrenAlign : "center",
	backgroundImage : centerImg
});
 
var barRight = new qx.ui.layout.CanvasLayout;
barRight.set({
	width: 5,
	height : 26,
	backgroundImage : rightImg
});
 
bar.add(barLeft, barCenter, barRight);