css on iframe not working

Question

Can anyone tell me why css stylings are not working on the iframe. I tried using both inline as well as external css

    iframe{
       height: 100%; border:2px solid black; 
    } 

    <link rel="stylesheet" href="new.css">
   <iframe class="fr" id="contentt" name="contentt"></iframe>
   
  
    <li><a href="t.php" target="content">Career Profiling Test</a></li>
    <li><a href="app.php" target="content">Schedule Appointment</a></li>
    <li><a href="out.php" target="content">Sign Out</a></li>
    </ul>
   

My Php Looks Like

session_start();
if(isset($_SESSION['signed_in']) && $_SESSION['signed_in'] == true)
{
  echo'<h3 style="color:green; text-align:right;">WELCOME 
 '.$_SESSION['first_name'].'</h3>';
}
echo '<ul class="ver">';