Container Context!

mytheme.theme.php

<?php

use Drupal\Core\Render\Element;

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function THEME_theme_suggestions_container_alter(array &$suggestions, array $variables) {
if (!empty($variables['element'])) {
    $element = $variables['element'];
  }

  // Provide suggestions to create context, if none already exist.
  if (empty($suggestions)) {
    // If there are no suggestions the implementation is likely via
    // #theme_wrappers, which doesn't provide any context. In that case, use the
    // element being wrapped to derive better context.
    if (!empty($element['#theme_wrappers'])) {
      if (in_array('container', $element['#theme_wrappers'])) {
        // If #type and #theme_wrappers are both 'container', avoid making the
        // context situation worse.
        if (!empty($element['#type']) && $element['#type'] == 'container') {
          // The children elements likely use #theme, which tells us what is
          // being rendered. This will capture things like the views more link:
          // container__more_link.
          foreach (Element::children($element) as $key) {
            if (!empty($element[$key]['#theme'])) {
              if (is_array($element[$key]['#theme'])) {
                // If #theme is an array, capture all values.
                // @todo Maybe it's enough to just use the first or last.
                foreach ($element[$key]['#theme'] as $theme_key => $theme_value) {
                  $suggestions[] = 'container__' . $theme_value;
                }
              }
              else {
                $suggestions[] = 'container__' . $element[$key]['#theme'];
              }
            }
          }
        }
        elseif (!empty($element['#type'])) {
          // As long as #type is not also 'container', it's just as good an
          // indication of context as #theme. This will capture a Views
          // element container, e.g. container__view
          $suggestions[] = 'container__' . $element['#type'];
        }
      }

        // If there is a #name property, this can be useful too. For example,
        // Views module uses this hook, and this would generate a suggestion
        // like: container__view__machinename.
        if (!empty($element['#name']) && !empty($element['#type'])) {
          $suggestions[] = 'container__' . $element['#type'] . '__' . $element['#name'];
        }
      }
    }

  // Re-route some templates based on those smarter suggestions.
  if (in_array('container__view', $suggestions)) {
    $suggestions[] = 'container__bare';
  }
}