The $tbData->isHTTPS variable is set from tb_themes\shoppica2\library\Utils.php -> public static function isRequestHTTPS(). This is the recommended way of detecting secure pages, but you can change the code if you like. For example you can try to replace the function with:

    public static function isRequestHTTPS()
    {
        $isSecure =
            (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off')
            || (!empty($_SERVER['SERVER_PORT']) && intval($_SERVER['SERVER_PORT']) == 443);

        if ((!empty($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https')
            || !empty($_SERVER['HTTP_X_FORWARDED_SSL']) && $_SERVER['HTTP_X_FORWARDED_SSL'] == 'on') {
            $isSecure = true;
        }

        return $isSecure;
    }

If this does not work, you can try with custom implementation. Please, not that Shoppica2 is not supported anymore.

Regards,
Themeburn Team