add a background class to body or document to change color code example

Example 1: javascript style background color

// change background color for specific id ..
function changebackground(){
	document.getElementById('id').style.backgroundColor = 'green' ; 
}
// change background color for whole body..
function changebackground(){
	document.body.style.backgroundColor = 'green';
}

Example 2: how to change background color using js

var warning = document.getElementById("warning");
  warning.style.background-color = "red";