Hello
In your templates styles.php (../templates/j51_virtuoso/php/styles.php) try replacing the following (approx. line 21)...
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1"/>
With...
<meta content="width=device-width; initial-scale=1.0; maximum-scale=2.0; user-scalable=1;" name="viewport">
Ciarán