demi379,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css"> .black {
background-color: #000000;
color: #FFFFFF;
}
</style>
<script>
document.addEventListener( "DOMContentLoaded" , function() {
let start = document.querySelector('button');
let num = 0;
start.addEventListener('click', function () {
let btn = document.createElement('button');
document.body.appendChild(btn)
btn.innerHTML = `hi ${++num}`;
btn.id = `first${num}`;
btn.addEventListener('click', function () {
let btn = document.createElement('button');
document.body.appendChild(btn)
btn.innerHTML = `hi ${++num}`;
btn.id = `second${num}`;
btn.classList.add('black');
});
});
});
</script>
</head>
<body>
<button>button 0</button>
</body>
</html>