body {
    background: url(../Estilos/fondo.jpg) no-repeat center center fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}

    body::before {
        content: "";
        position: absolute;
        top: 0px;
        right: 0px;
        bottom: 0px;
        left: 0px;
        background-color: rgba(0,0,0,0.45);
        z-index:-1;
    }

.forumarioCentrado {
    width: 500px;
    margin: auto;
    max-width: 800px;
}

.forumarioCentradog {
    width: 700px;
    margin: auto;
    max-width: 1000px;
}