Useful theme suggestions (may use deprecated php)

drupal-theme-suggestions.theme

<?php

use Drupal\block\Entity\Block;
use Drupal\block_content\BlockContentInterface;

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function HOOK_theme_suggestions_page_alter(array &$hooks, array &$vars): void {
  $constrained = [
    'page__user',
    'page__sitemap',
    'page__404',
    'page__403',
    'page__node__generate_preview_link',
    'page__node__layout__discard_changes',
    'page__node__layout__revert',
  ];
  if (array_intersect($constrained, $hooks)) {
    $hooks[] = 'page__constrained';
  }
}

/**
 * Implements hook_theme_suggestions_block_alter().
 */
function HOOK_theme_suggestions_block_alter(array &$hooks, array &$vars): void {
  $original = $vars['theme_hook_original'];
  $view_mode = null;

  // View mode
  if (isset($vars['elements']['#configuration']['view_mode'])) {
    $view_mode = $vars['elements']['#configuration']['view_mode'];
    $hooks[] = "{$original}__{$view_mode}";
  }

  if (isset($vars['elements']['#id'])) {
    $block_id = $vars['elements']['#id'];
    if (($block = Block::load($block_id)) && $block instanceof Block) {

      // Region
      $region = $block->getRegion();
      $hooks[] = "{$original}__{$region}__{$vars['elements']['#base_plugin_id']}";
      $hooks[] = "{$original}__{$region}__{$block_id}";

      // Pass region through to hook_preprocess_block().
      $vars['elements']['#region'] = $region;

      // Custom theme_suggestions based on ID
      switch ($block_id) {
        case 'mainpagecontent':
        case 'pagetitle':
          $hooks[] = "{$original}__bare";
          break;
      }
    }
  }

  // Custom Blocks (Bundles)
  if (isset($vars['elements']['content']['#block_content'])) {

    if (($block = $vars['elements']['content']['#block_content']) && $block instanceof BlockContentInterface) {
      // Bundle type
      if ($bundle = $block->bundle()) {
        $hooks[] = "{$original}__{$bundle}";
        if ($view_mode) {
          $hooks[] = "{$original}__{$bundle }__{$view_mode}";
        }

        // Pass bundle through to hook_preprocess_block().
        $vars['elements']['#bundle'] = $bundle;
      }
    }
  }

  // Custom theme suggestions based on plugin.
  $plugin_id = $vars['elements']['#plugin_id'] ?? '';
  if (str_starts_with($plugin_id, 'system_menu_block:footer-')) {
    $hooks[] = "{$original}__footer_menu";
  }
}

/**
 * Implements hook_theme_suggestions_field_alter().
 */
function HOOK_theme_suggestions_field_alter(array &$hooks, array &$vars): void {
  $original = $vars['theme_hook_original'];
  $name = $vars['element']['#field_name'];

  // View mode
  if ($vars['element']['#view_mode']) {
    $view_mode = $vars['element']['#view_mode'];
    $hooks[] = "{$original}__{$view_mode}";
    $hooks[] = "{$original}__{$view_mode }__{$name}";
  }

  // Name
  $bare_names = [
    'body',
    'title',
    'components',
  ];
  // Hook
  $bare_hooks = [
    'media:image:hero:field_name',
    'node:news:teaser:field_name',
  ];
  $hook = implode(':', [
    $vars['element']['#entity_type'],
    $vars['element']['#bundle'],
    $vars['element']['#view_mode'],
    $vars['element']['#field_name'],
  ]);

  // Suggest keeping field.html.twig very sparce of markup,
  // then field--bare.html.twig has absolutely none.
  if (in_array($name, $bare_names, TRUE) || in_array($hook, $bare_hooks, TRUE)) {
    $hooks[] = "{$original}__bare";
  }
}

/**
 * Implements hook_theme_suggestions_menu_alter().
 */
function HOOK_theme_suggestions_menu_alter(array &$hooks, array &$vars): void {
  $original = $vars['theme_hook_original'];

  // These are present as we set them in hook_preprocess_block().
  // @see https://gist.github.com/rikki-iki/f07f9a741597dfabecfd6f88ba69d435#file-drupal-8-preprocess-theme-L94

  if (isset($vars['attributes']['region'])) {
    $hooks[] = "{$original}__{$vars['attributes']['region']}";
  }
  if (isset($vars['attributes']['block_id'])) {
    $block_id = str_replace(array('block-', '-'), array('', '_'), $vars['attributes']['block_id']);
    $hooks[] = "{$original}__{$block_id}";
  }
  if (preg_match('/^footer-/', $vars['menu_name'])) {
    $hooks[] = "{$original}__footer";
  }
}

/**
 * Implements hook_theme_suggestions_user_alter().
 */
function HOOK_theme_suggestions_user_alter(array &$hooks, array &$vars): void {
  $original = $vars['theme_hook_original'];
  $view_mode = $vars['elements']['#view_mode'];
  $hooks[] = "{$original}__{$view_mode}";
}

/**
 * Implements hook_theme_suggestions_taxonomy_term_alter().
 */
function HOOK_theme_suggestions_taxonomy_term_alter(array &$hooks, array &$vars): void {
  $original = $vars['theme_hook_original'];
  $term = $vars['elements']['#taxonomy_term'];
  $hooks[] = "{$original}__{$term->bundle()}__{$vars['elements']['#view_mode']}";
}

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function HOOK_theme_suggestions_form_element_alter(array &$hooks, array &$vars): void {
  $original = $vars['theme_hook_original'];
  if ($vars['element']['#type'] === 'select') {
    $hooks[] = "{$original}__select";
  }
}

/**
 * Implements hook_theme_suggestions_media_alter().
 */
function HOOK_theme_suggestions_media_alter(array &$hooks, array &$vars): void {
  $original = $vars['theme_hook_original'];
  $view_mode = $vars['elements']['#view_mode'] ?? 'default';
  $bundle = $vars['elements']['#bundle'] ?? $vars['elements']['#media']->bundle->entity->id();
  $bare_list = [
    'hero',
    'full',
  ];
  if (in_array($view_mode, $bare_list, TRUE) || $bundle === 'file') {
    $hooks[] = "{$original}__bare";
  }
}

/**
 * Implements hook_theme_suggestions_views_view_unformatted_alter().
 */
function HOOK_theme_suggestions_views_view_unformatted_alter(array &$hooks, array &$vars): void {
  $original = $vars['theme_hook_original'];
  /** @var \Drupal\views\ViewExecutable $view */
  $view = $vars['view'];
  $bare_list = [
    'event_listing:event_listing_block',
    'event_listing:past_event_listing_block',
    'news_listing:news_listing_block',
  ];
  if (in_array(sprintf('%s:%s', $view->id(), $view->current_display), $bare_list, TRUE)) {
    $hooks[] = "{$original}__bare";
    $hooks[] = "{$original}__{$view->id()}";
    $hooks[] = "{$original}__{$view->id()}__{$view->current_display}";
  }
}

This was super helpful, thanks!

Really helpful, especially the theme suggestions for Custom Blocks (Bundles), thanks!