html,body {
    font-size:14px;
    margin:0;
    background:  url(images/fondo.jpg) no-repeat center center fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover; 
}

.contenedor {
    margin:0 auto;
    width:800px;
    text-align:center;
}
span {
    color:red;
}

#formulario {
    margin:0 auto;
    border-radius:5px;
    box-shadow:2px 3px 4px #000;
    border:1px solid #000;
    width:300px;
    padding:25px;
    background:#d1d1d1;
}

input {
    border-radius:5px;
    padding:8px;
}

button {
    border-radius:5px;
    padding:8px;
    background:#000;
    color:#fff;
    cursor:pointer;
}