Transparent list box

//lbDemo is a ListBox control.
    protected void lbDemo_SelectedIndexChanged(object sender, EventArgs e)
    {
        setItem(lbDemo, lbDemo.SelectedIndex);
    }

    void setItem(ListBox lb,int SelectedIndex)
    {
        foreach (ListItem li in lb.Items)
        {
            li.Attributes.Add("style", "color:black");
        }
        lb.Items[SelectedIndex].Attributes.Add("style", "color:blue");
    }