CSS selector with :

HTML

<link rel="stylesheet" href="https://cdn.jsdelivr.net/chartist.js/latest/chartist.min.css">
<script src="https://cdn.jsdelivr.net/chartist.js/latest/chartist.min.js"></script>
<div id="myChart" class="ct-chart" style="height:400px"></div>

CSS

/* This works:
 #myChart .ct-series-b .ct-bar { 
 */


/* This does not (chroium, glnxa64) */
['ct:series-name'='second'] .ct-bar {
  /* Colour of your points */
  stroke: black;
  /* Size of your points */
  stroke-width: 20px;
  /* Make your points appear as squares */
  stroke-linecap: round;
}

JavaScript

var data = {
  labels: ['W1', 'W2', 'W3', 'W4', 'W5', 'W6', 'W7', 'W8', 'W9', 'W10'],
  series: [{
    name: 'first',
    data: [1, 2, 4, 8, 6, -2, -1, -4, -6, -2]
  }, {
    name: 'second',
    data: [3, 4, 2, 6, 3, 2, 1, 4, 6, 2]
  }]
};

var options = {
  high: 10,
  low: -10,
  onlyInteger: true
};

new Chartist.Bar('.ct-chart', data, options);