document.body.className code example

Example 1: js add class

var element = document.getElementById('element');
element.classList.add('class-1');
element.classList.add('class-2', 'class-3');
element.classList.remove('class-3');

Example 2: html get class property

let element = document.querySelector('.preset');
let style = getComputedStyle(element);
let bg = style.backgroundColor;