<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
body{
background-color: black;
color: white;
}
div{
border: 1px solid gray;
transition: 1s;
}
.cl{
color:black;
}
</style>
</head>
<body>
<div>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Pariatur, repudiandae molestias repellat sint dignissimos nostrum ut. Voluptate vitae quisquam, in nobis dolorum, iste laboriosam. Ex dolores, neque quis ea laudantium!</div>
<button>Затемнить постепенно</button>
<script>
document.querySelector('button').onclick=e=>document.querySelector('div').className='cl';
</script>
</body>
</html>
пфф... неужели таки везде??
