Untitled

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>some Html template</title>
    <link type="text/css" rel="stylesheet" href="css/app.css">
</head>
<body>
    <header></header>
    <main>
        <h1>Content</h1>
    </main>
    <footer></footer>


    <div id="overlay" style="display: block;">
        <div>
            Для продолжения нажмите кнопку<br>
            <button>Далее</button>
        </div>
    </div>
</body>
</html>


html, body {
    margin: 0;
    padding: 0;
    font-size: 15px;
    font-family: "American Typewriter";
    background: aliceblue;
}
div {
    box-sizing: border-box;
}

header {
    height: 80px;
    background: red;
}

main {
    height: 500px;
    background: yellow;
}

footer {
    height: 50px;
    background: black;
}

h1  {
    margin: 0;
}


#overlay {
    position: fixed;
    left: 0;
    right: 0;
    bottom: 0;
    top: 0;
    background: rgba(0,0,0,0.4 );
}
#overlay div {
    margin: 20% auto;
    width: 30%;
    padding: 30px;
    text-align: center;
    background: white;
}