How to change CSS property using JavaScript

You can use style property for this. For example, if you want to change border -

document.elm.style.border = "3px solid #FF0000";

similarly for color -

 document.getElementById("p2").style.color="blue";

Best thing is you define a class and do this -

document.getElementById("p2").className = "classname";

(Cross Browser artifacts must be considered accordingly).

// select element from DOM using *const*
const sample = document.getElementById("myid"); // using CONST
// or you can use *var*
var sample = document.getElementById("myid"); // using VAR

// change css style
sample.style.color = 'red'; // Changes color, adds style property.
// or (not recomended)
sample.style = "color: red"; //Replaces all style properties. NOT RECOMENDED

Use document.getElementsByClassName('className').style = your_style.

var d = document.getElementsByClassName("left1");
d.className = d.className + " otherclass";

Use single quotes for JS strings contained within an html attribute's double quotes

Example

<div class="somelclass"></div>

then document.getElementsByClassName('someclass').style = "NewclassName";

<div class='someclass'></div>

then document.getElementsByClassName("someclass").style = "NewclassName";

This is personal experience.