body{
    text-align: center;
    font-family: sans-serif;
}

.modal{
    width: 100%;
    height: 100vh;
    z-index: 100;
    background: rgba(0,0,0,0.9);
    top: 0;
    left: 0;
    display: flex;;
    position: fixed;
}
.contenido{
    max-width: 100%;
    max-height: 100%;
    margin: auto;
    display: block;
    }

#cerrar{
    display: none;
}
        #cerrar + label{
            position: fixed;
            color: #fff;
            font-size: 25px;
            z-index: 101;
            background: darkred;
            height: 40px;
            width: 40px;
            line-height: 40px;
            border-radius: 50%;
            right: 10px;
            cursor: pointer;
            text-align: center;        
        }
#cerrar:checked + label, #cerrar:checked ~ .modal{
    display: none;
}