Hi,
In that same file, replace all the content between the "Buttons" label (lines #21 to #36) with this new one:
function webtreats_button( $atts, $content = null ) {
extract(shortcode_atts(array(
'link' => '#',
'blank' => 'false'
), $atts));
$blank_link = '';
if ( $blank == 'true' )
$blank_link = "target=\"_blank\"";
$out = "<a class=\"button_link\" href=\"" .$link. "\"" .$blank_link."><span>" .do_shortcode($content). "</span></a>";
return $out;
}
add_shortcode('button', 'webtreats_button');
Now, if you want the button to open in a new tab, you have to add the attribute "blank" to the shortcode, and set it's value to "true".
For example:
[button link="google.com" blank="true"]...[/button]
Try that and let me know the results.