Edit in JSFiddle 

function addAddr() {
	const result = document.getElementById('addrlist');
  const newAddr = document.getElementsByName("name")[0].value + ' , ' + document.getElementsByName('tel')[0].value;
    const item = document.createElement('li');
    const txt = document.createTextNode(newAddr);
    item.appendChild(txt);
    result.appendChild(item);
}


function resetAddr() {
	document.getElementById('addrlist').innerHTML="";
}

function changeBg() {
	body=document.querySelector('body');
  body.setAttribute('style','background-color:yellow');
  //body.style.backgroundColor="yellow";
 // body.className="bgyellow";
}
<html>
  <body>
    <h2>주소록 예제</h2>
    <form>
      이름: <input type="text" name="name"><br> 전화번호: <input type="text" name="tel"><br>
      <input type=button value="저장" onclick='addAddr()'>
      <input type=button value="리셋" onclick='resetAddr()'>
      <input type=button value="배경변경" onclick='changeBg()'>
    </form>
    <hr>
    <h3>주소록 목록</h3>
    <ul id="addrlist">

    </ul>
  </body>
</html>

body {
  background-color:white;
}

.bgyellow {
  background-color:yellow;
}

External resources loaded into this fiddle: