in the WA_Universal_Email/Templates/contactus.php file,you could change:
<h1 style="color: #777890;font-size: 14px;font-weight: bold;margin:0;">Contact Form Response</h1>
to:
<h1 style="color: #fff;font-size: 14px;font-weight: bold;margin:0;">Contact Form Response</h1>