Hello,
When you copied the Mobile.css file, please make sure the HTML template is pointing to the new Mobile.css file.
However, since the autogenerated code in the template already points to the Mobile.css file, it is better you add new classes in this existing file itself and change the HTML template classes for tags:
<body class="MobileBodyBig" .....> </body>
Add in Mobile.css:
MobileBodyBig{
height:100%;
width: 100%;
font-size:16px;
}
Remember to save and publish Completely - this mobile.css as well as the HTML template.