You should use onclick method because the function run once when the page is loaded and no button will be clicked then
So you have to add an even which run every time the user press any key to add the changes to the div background
So the function should be something like this
htmlelement.onclick() = function(){
//Do the changes
}
So your code has to look something like this :
var box = document.getElementById("box");
var yes = document.getElementById("yes");
var no = document.getElementById("no");
yes.onclick = function(){
box.style.backgroundColor = "red";
}
no.onclick = function(){
box.style.backgroundColor = "green";
}
This is meaning that when #yes button is clicked the color of the div is red and when the #no button is clicked the background is green
Here is a Jsfiddle