body{
    background-image: url(imagem.jpg);
    background-repeat: no-repeat;
    background-position: center;
    background-attachment: fixed;
    background-size: cover;
}

.center{
    font-size: 50px;
    color: yellow;
    text-align: center;
    margin: 0;
    position: absolute;
    top: 50%;
    left: 50%;
    margin-right: -50%;
    transform: translate(-50%, -50%)
}