How to send variables to a custom theme from a custom module

I'm trying to pass a variable from a custom module to a custom theme. I have a Controller class in a custom module

<?php

namespace Drupal\course_catalog\Controller;

use Drupal\Core\Controller\ControllerBase;

Class CourseCatalog extends ControllerBase {
  public function build() {

    $item = 'Test content';

    return [
      '#theme' => 'region__content',
      '#item' => $item,
    ];
  }
}

And this hook_theme() in the .module

<?php

/**
 * Implements hook_theme().
 */
function course_catalog_theme() {
  return [
    'region__content' => [
      'variables' => ['item' => NULL]
    ]
  ];
}

Lastly I have my custom theme I have region--content.html.twig

<h2>Content: {{ item }}</h2>

This item value never render on the page. Have I missed something here ?