This is what we have:

Code:
  // HTTP_SERVER is your Main webserver: eg-http://www.your_domain.com
  // HTTPS_SERVER is your Secure webserver: eg-https://www.your_domain.com
  define('HTTP_SERVER', 'https://launchingsuccess.com/store');
  define('HTTPS_SERVER', 'https://www.launchingsuccess.com/store');

  // Use secure webserver for checkout procedure?
  define('ENABLE_SSL', 'true');

On our DEV site, when I enable any of the other themes, they all work, but with the currant theme, it doesn't.. so it appears to very definitely be in the theme that our old developer created.. But as this is the first time I've used ZenCart specifically, I'm not 100% on where to be looking to fix it.