Getting or changing CSS class property with Javascript using DOM style

I think this is not the best way, but in my cases other methods did not work.

stylesheet = document.styleSheets[0]
stylesheet.insertRule(".have-border { border: 1px solid black;}", 0);

Example from https://www.w3.org/wiki/Dynamic_style_-_manipulating_CSS_with_JavaScript

As mentioned by Quynh Nguyen, you don't need the '.' in the className. However - document.getElementsByClassName('col1') will return an array of objects.

This will return an "undefined" value because an array doesn't have a class. You'll still need to loop through the array elements...

function changeBGColor() {
  var cols = document.getElementsByClassName('col1');
  for(i = 0; i < cols.length; i++) {
    cols[i].style.backgroundColor = 'blue';
  }
}