Drupal 8 Block and Menu template suggestions

themename.theme.php

<?php

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function THEMENAME_theme_suggestions_block_alter(array &$suggestions, array $variables) {
  $block_id = $variables['elements']['#id'];
  // See if block ID contains the word mobile.
  $is_mobile_block = strpos($block_id, 'mobile');
  $block = \Drupal\block\Entity\Block::load($block_id);
  $region = $block->getRegion();

  // Check if block ID contains the word 'mobile' then define a common template.
  if ($is_mobile_block !== false) {
    $suggestions[] = 'block__' . $region . '__mobile_menus';
  }

}

/**
 * Implements hook_preprocess_HOOK() for block.html.twig.
 */
function THEMENAME_preprocess_block(&$variables) {
  // Add a clearfix class to system branding blocks.
  if ($variables['plugin_id'] == 'system_branding_block') {
    $variables['attributes']['class'][] = 'clearfix';
  }
  // Add the block ID as custom attribute to block content, this will be used
  // for menu template suggestions.
  $variables['content']['#attributes']['block'] = $variables['elements']['#id'];
}

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function THEMENAME_theme_suggestions_menu_alter(array &$suggestions, array $variables) {
  $menu_name = $variables['menu_name'];
  $is_mobile_menu = strpos($menu_name, 'mobile');
  if (isset($variables['attributes']['block'])) {
    $block = \Drupal\block\Entity\Block::load($variables['attributes']['block']);
    $region = $block->getRegion();
    $suggestions[] = 'menu__' . $region . '__' . $menu_name;
  }
  // If menu name contains the word mobile, create common template suggestion.
  if ((isset($variables['attributes']['block'])) && ($is_mobile_menu !== false)) {
    $suggestions[] = 'menu__' . $region . '__mobile-menus';
  }
}

/**
 * Implements hook_preprocess_HOOK() for menu.html.twig.
 */
function THEMENAME_preprocess_menu(&$variables) {
  $menu_name = $variables['menu_name'];
  $is_mobile_menu = strpos($menu_name, 'mobile');
  if ($is_mobile_menu !== false) {
    $variables['attributes']['class'][] = 'mobile-menu';  
  }
  $variables['attributes']['class'][] = 'clearfix';
}