D8: Theming examples: Theme: Additional suggestion for content block templates: taking the bundle type into account. *First results of struggling through Drupal 8 - better solutions are probably out there! **Drupal 8 is still Beta - improvements through API modifications are possible!

theme

<?php

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function MYTHEME_theme_suggestions_block_alter(array &$suggestions, array $variables) 
{
    $block = $variables['elements'];
    $blockType = $block['#configuration']['provider'];
    if ($blockType == "block_content") {
        $bundle = $block['content']['#block_content']->bundle();
        $blockUuid = $block['content']['#block_content']->uuid();
        $blockId = $block['#id'];
        $suggestions[] = 'block__' . $blockType . '__' . $bundle;
        $suggestions[] = 'block__' . $blockType . '__' . $bundle . '__' . $blockId;
        $suggestions[] = 'block__' . $blockType . '__' . $bundle . '__' . $blockUuid;
    }
}

zhelp

As Example

admin/structure/block/block-content/types

As custom block type is defined: "header_content_block":

<!-- FILE NAME SUGGESTIONS:
   new! block--block-content--header-content-block--7bd0dc95-f2ee-4e70-8a5a-90d718384712.html.twig
   new! block--block-content--header-content-block--registernowheaderblock.html.twig
   new! block--block-content--header-content-block.html.twig
   * block--registernowheaderblock.html.twig
   * block--block-content--7bd0dc95-f2ee-4e70-8a5a-90d718384712.html.twig
   * block--block-content.html.twig
   * block.html.twig
-->

The hierachy ist not perfect - no idea, how to fix ... better would be:

<!-- FILE NAME SUGGESTIONS:
   * block--registernowheaderblock.html.twig
   * block--block-content--7bd0dc95-f2ee-4e70-8a5a-90d718384712.html.twig
   new! block--block-content--header-content-block.html.twig
   * block--block-content.html.twig
   * block.html.twig
-->