adaptivetheme-8.x-3.x-dev/at_core/forms/ext/custom_css.php

<?php

/**
 * @file
 * Generate settings for the Custom CSS form.
 */

use Drupal\Component\Utility\Xss;

$form['custom-styles'] = [
  '#type' => 'details',
  '#title' => t('Custom CSS'),
  '#group' => 'extension_settings',
];

$form['custom-styles']['settings_custom_css'] = [
  '#type' => 'textarea',
  '#title' => t('Custom CSS'),
  '#rows' => 20,
  '#default_value' => theme_get_setting('settings.custom_css') ? Xss::filterAdmin(theme_get_setting('settings.custom_css')) : '/* Custom CSS */',
  '#description' => t("<p>Styles entered here are saved to <code>@theme_path/styles/css/generated/custom-css.css</code>.</p><p>Manual changes to the generated file are overwritten when submitting this form. Do not use this for building your entire theme - instead modify your themes CSS component stylesheets or use custom.css: <code>styles/css/custom.css</code>.</p><p>Note: for security reasons you cannot use the greater than symbol (>) as a child combinator selector.</p>", ['@theme_path' => $subtheme_path]),
];