Edit in JSFiddle

var element = document.getElementById('test');

function toggle() {
	element.classList.toggle('test-class');
}

function add() {
	element.classList.add('test-class');
}

function remove() {
	element.classList.remove('test-class');
}

<div id="test" style="background: #FF0000; width: 30px; height: 30px">div</div>
<button onclick="toggle()">
Toggle
</button>

<button onclick="add()">
Add
</button>

<button onclick="remove()">
Remove
</button>

.test-class {
  background: #00FF00 !important
}