admincss-8.x-2.2/admincss.module

<?php

/**
 * @file
 * Hook implementations for the Admin CSS module.
 */

use Drupal\Core\Cache\CacheableMetadata;
use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Implements hook_help().
 */
function admincss_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.admincss':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Admin CSS module provides a way to add custom CSS to the site.') . '</p>';
      return $output;
  }
}

/**
 * Implements hook_page_attachments().
 */
function admincss_page_attachments(array &$attachments) {
  $theme = \Drupal::theme()->getActiveTheme()->getName();
  $default_theme = \Drupal::config('system.theme')->get('default');
  if ($default_theme === $theme) {
    if (file_exists('public://admin-style.css')) {
      $attachments['#attached']['library'][] = $theme . '/admincss.admincss';
    }

    // Add the admin css cache tags.
    $admincss_config = \Drupal::config('admincss.settings');
    $cacheable_metadata = CacheableMetadata::createFromObject($admincss_config);
    $cacheable_metadata->applyTo($attachments);
  }
}

/**
 * Implements hook_library_info_alter().
 */
function admincss_library_info_alter(&$libraries, $extension) {
  $theme = \Drupal::theme()->getActiveTheme()->getName();
  if ($theme === $extension) {
    if (!isset($libraries['admincss.admincss']['css'])) {
      // Add the library to the active theme with a relatively high weight.
      $libraries['admincss.admincss']['css']['theme']['public://admin-style.css']['weight'] = 9999;
    }
  }
}