Error,
как вариант ...
<!doctype html>
<html lang="ru">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
html.active .attention{
height: 100px;
background-color: #4047CC;
}
</style>
<script>
let d = new Date();
document.documentElement.classList.toggle('active', d.getHours() < 9)
</script>
</head>
<body>
<div class="attention"></div>
</body>
</html>