<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
body{
transition: 2s;
}
</style>
</head>
<body>
Цвет фона
<select name="" id="fon">
<option value="white">white</option>
<option value="red">red</option>
<option value="green">green</option>
<option value="blue">blue</option>
</select>
<script>
document.getElementById('fon').onchange= e => document.body.style.backgroundColor = e.target.value;
</script>
</body>
</html>