set css background from javascript 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: js background image

document.body.style.backgroundImage = "url('img_tree.png')";