Set style using pure JavaScript

document.getElementsByTagName('div')[0].style.backgroundColor = 'RED';

<div>sample</div>

There are many ways you can set the background color. But getElementsByTagName does not return a single object. It's a collection of objects

document.body.style.backgroundColor = "green"; // JavaScript

document.getElementsByTagName("body")[0].style.backgroundColor = "green"; // Another one

See the demo