Why can't I pass this variable to a Twig view?

I am working on a online newspaper/blogging application with CodeIgniter 3.1.8 and Bootstrap 4. I have decided to add themes to it. The application is not HMVC, only MVC.

The themes directory is outside application as can be see in the image below:

Inside themes I have the theme directory (of course) which contains the "master view", 

Code:

layout.php

:

How I use the theme views

In application/core I have added a MY_Loader.php file with the following contents:

Code:

<?php defined('BASEPATH') OR exit('No direct script access allowed');

class MY_Loader extends CI_Loader {

function theme_view($folder, $view, $vars = array(), $return = FALSE) {
  $this->_ci_view_paths = array_merge($this->_ci_view_paths, array(FCPATH . $folder . '/' => TRUE));
   return $this->_ci_load(array(
    '_ci_view' => $view,
    '_ci_vars' => $this->_ci_prepare_view_vars($vars),
    '_ci_return' => $return
   ));
  }
}

In my Posts controller's index() method, I load the view passing it the data:

Code:

public function index() {
  //more code here
  $data['posts'] = $this->Posts_model->get_posts($config['limit'], $config['offset']);
  $this->load->theme_view('/themes/caminar/', 'layout', $data);
}

Using Twig for the themes

The next step was to add the [i]Twig[/i] template engine to the theme(s). For this purpose, I use CodeIgniter Simple and Secure Twig. I have:

• Added the [i]Simple and Secure Twig[/i] library in application\libraries
  
  
  
•  In the third_party directory I have added Twig itself.
  
  
  
• In application\config\autoload.php I have loaded twing:
  $autoload['libraries'] = array('database', 'form_validation', 'session', 'user_agent', 'twig');
  
  
• Set the path for the Twig views in application\libraries\Twig.php:
  

Code:

private $config = [ 'paths' => [FCPATH . '/themes', VIEWPATH], 'cache' => '/path/to/twig/cache', ];

The problem

According to the docs of CodeIgniter Simple and Secure Twig, we can set a global variable like so: 

Code:

$this->twig->addGlobal('sitename', 'My Awesome Site');

So, I added 

Code:

$this->twig->addGlobal('siteTitle', 'My Awesome Site');

 in the Posts controller:

Code:

public function index() {

//call initialization method
$config = $this->_initPagination("/", $this->Posts_model->get_num_rows());

$data = $this->Static_model->get_static_data();
$data['pages'] = $this->Pages_model->get_pages();
$data['categories'] = $this->Categories_model->get_categories(); 

//use limit and offset returned by _initPaginator method
$data['posts'] = $this->Posts_model->get_posts($config['limit'], $config['offset']);
$this->twig->addGlobal('siteTitle', 'My Awesome Site');
$this->load->theme_view('/themes/caminar/', 'layout', $data);
}

Yet, in themes\caminar\layout.php the line 

Code:

<title>{{siteTitle}}</title>

 
does not display "My Awesome Site" inside the <title> tag.
What am I doing wrong?