HTML
<!DOCTYPE html>
<html lang="pt-br">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<link rel="stylesheet" href="mecentraliza.css">
</head>
<body>
<main class="centralizar">
<div>
<p>Me centraliza!</p>
</div>
</main>
</body>
CSS
main {
height: 100vh;
width: 100vw;
}
.centralizar {
display: flex;
justify-content: center;
align-items: center;
}
div {
width: 240px;
height: 110px;
font-size: 36px;
font-weight: bold;
background-color: #fff;
border-radius: 4px;
padding: 0 10px;
background-image: linear-gradient(315deg, #4dccc6 0%, #96e4df 74%);
box-shadow: 4px 4px 6px 0 rgba(255, 255, 255, .3),
-4px -4px 6px 0 rgba(116, 125, 136, .2),
inset -4px -4px 6px 0 rgba(255, 255, 255, .2),
inset 4px 4px 6px 0 rgba(0, 0, 0, .2);
}