.tab {
    display: inline-block;
    margin-left: 4em;
}

#mdiv{
    margin-right: auto; 
    width: 40%;
    font-size: 16pt;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    margin-left: auto;
    background-color: whitesmoke;
    padding: 75px;
    border: 1px solid black;

}

/* unvisited link */
a:link {
    color: dimgray;
  }
  
/* visited link */
a:visited {
    color: dimgray;
  }
  
/* mouse over link */
a:hover {
    color: black;
  }
  
/* selected link */
a:active {
    color: dimgray;
  }

body {
    background-color: grey;
}

#ct {
    text-align:center;
}

only screen and (max-device-width: 480px) {
  body {background:red;}
}