Drupal 8 Commerce Module Hook_Theme() I'm working on a custom module that I want to hook into a custom commerce template. 

Heres is the code inside of my custom module the returns the variable. 

 /**
     * Implements hook_theme().
     * @param $existing
     * @param $type
     * @param $theme
     * @param $path
     * @return array
     */
    function my_theme($existing, $type, $theme, $path) {
        return [
              'commerce-product--full' =&gt; [
                'variables'=&gt; array (
                    'key'=&gt; "I'm a key.",
                ),
              ],
        ]
}


Inside of commerce-product--full, using kint(key), I get null as a response. 

Am I missing something?
Try implementing hook_theme_registry_alter() and see how that goes.

function mymodule_theme_registry_alter(&amp;$theme_registry) {
    $theme_registry['commerce_product__full']['path'] = drupal_get_path('module', 'mymodule') . '/templates';
}


Your module presumably will have a folder named templates. And inside the folder you have a file called commerce-product--full.html.twig