if you only want to change some colours , fonts , paddings and a few simple other things, you might be able to use the following filter in your functions.php – instead of editing the template directly –

add_filter('wppizza_filter_html_email_style', my_custom_style);
function my_custom_style($htmlEmailStyle){
/**change styles as required - print_r($htmlEmailStyle) to see all vars -  for example: **/
$htmlEmailStyle['pageBackgroundColor']='green';
$htmlEmailStyle['textColour']='#FFFFFF';
return $htmlEmailStyle;
}