[Solved]-Border Styles for the Highchart tooltip-Highcharts

You simply use an HTML tooltip in Highcharts by setting useHTML: true; see example: http://jsfiddle.net/xq28t/

In JS:

tooltip: {
    useHTML: true,
    borderWidth: 0,
    style: {
        padding: 0
    }
},

In CSS:

.highcharts-tooltip>span {
    padding: 10px;
    border-bottom: 1px solid black;
}