This is enhancement over https://www.drupal.org/project/wysiwyg_template/issues/2855470#comment-13157946

allow-to-add-css-for-editor-2855470-wwm.patch

diff --git a/config/install/wysiwyg_template.settings.yml b/config/install/wysiwyg_template.settings.yml
index 8b3a650..b2380fd 100644
--- a/config/install/wysiwyg_template.settings.yml
+++ b/config/install/wysiwyg_template.settings.yml
@@ -1 +1,3 @@
 library_path: 'libraries/templates'
+editor_css: ''
+view_css: ''
diff --git a/config/schema/wysiwyg_template.schema.yml b/config/schema/wysiwyg_template.schema.yml
index 46c694a..857962c 100644
--- a/config/schema/wysiwyg_template.schema.yml
+++ b/config/schema/wysiwyg_template.schema.yml
@@ -5,6 +5,12 @@ wysiwyg_template.settings:
     library_path:
       type: path
       label: 'Root-relative path of the CKEditor Templates plugin directory'
+    editor_css:
+      type: text
+      label: 'CSS to be used in editor'
+    view_css:
+      type: text
+      label: 'CSS to be used in content view'
 
 wysiwyg_template.wysiwyg_template.*:
   type: config_entity
diff --git a/src/Form/SettingsForm.php b/src/Form/SettingsForm.php
new file mode 100644
index 0000000..816ba3b
--- /dev/null
+++ b/src/Form/SettingsForm.php
@@ -0,0 +1,122 @@
+<?php
+
+namespace Drupal\wysiwyg_template\Form;
+
+use Drupal\Core\Form\ConfigFormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\File\FileSystemInterface;
+use Drupal\Core\Messenger\MessengerInterface;
+use Symfony\Component\Finder\Finder;
+
+/**
+ * WYSIWYG Template settings.
+ */
+class SettingsForm extends ConfigFormBase {
+  /** @var string Config settings */
+  const SETTINGS = 'wysiwyg_template.settings';
+
+  const CSS_FILE_DIRECTORY = 'public://mce';
+
+  const EDITOR_CSS_FILE_NAME = 'editor.css';
+
+  const VIEW_CSS_FILE_NAME = 'view.css';
+
+  public function getFormId() {
+    return 'wysiwyg_template_settings';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getEditableConfigNames() {
+    return [
+      static::SETTINGS,
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $config = $this->config(static::SETTINGS);
+
+    $form['editor_css'] = [
+      '#type' => 'textarea',
+      '#title' => $this->t('Editor CSS'),
+      '#default_value' => $config->get('editor_css'),
+      '#description' => $this->t('CSS to be used for WYSIWYG templates in editor.'),
+    ];
+
+    $form['view_css'] = [
+      '#type' => 'textarea',
+      '#title' => $this->t('View CSS'),
+      '#default_value' => $config->get('view_css'),
+      '#description' => $this->t('CSS to be used when WYSIWYG template are being used on content view pages.'),
+    ];
+
+    return parent::buildForm($form, $form_state);
+  }
+
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    $directory = static::CSS_FILE_DIRECTORY;
+    if (! \Drupal::service('file_system')->prepareDirectory($directory, FileSystemInterface::CREATE_DIRECTORY | FileSystemInterface::MODIFY_PERMISSIONS)) {
+      $form_state->setErrorByName('editor_css', t('CSS content cannot be written to file.'));
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    // Retrieve the configuration
+     $this->configFactory->getEditable(static::SETTINGS)
+    // Set the submitted editor CSS setting
+    ->set('editor_css', $form_state->getValue('editor_css'))
+    ->set('view_css', $form_state->getValue('view_css'))
+    ->save();
+
+    if ($new_editor_css_file_name = static::regenerateCSSFile('editor', $form_state->getValue('editor_css'))) {
+      \Drupal::state()->set('wysiwyg_template.editor_css_file', $new_editor_css_file_name);
+      $message = $this->t('Editor CSS has been written to file "@file."', ['@file'=> $new_editor_css_file_name]);
+      $status = MessengerInterface::TYPE_STATUS;
+    }
+    else {
+      $message = $this->t('Editor CSS could not be written to file.');
+      $status = MessengerInterface::TYPE_ERROR;
+    }
+    \Drupal::messenger()->addMessage($message, $status);
+
+    if (\Drupal::service('file_system')->saveData($form_state->getValue('view_css'), static::getViewCSSFilePath(), FileSystemInterface::EXISTS_REPLACE)) {
+      $message = $this->t('View CSS has been written to file.');
+      $status = MessengerInterface::TYPE_STATUS;
+    }
+    else {
+      $message = $this->t('View CSS could not be written to file.');
+      $status = MessengerInterface::TYPE_ERROR;
+    }
+    \Drupal::messenger()->addMessage($message, $status);
+
+    parent::submitForm($form, $form_state);
+  }
+
+  public static function  getViewCSSFilePath() {
+    return self::CSS_FILE_DIRECTORY . '/' . self::VIEW_CSS_FILE_NAME;
+  }
+
+  public static function regenerateCSSFile($prefix, $content) {
+    $new_file_name = $prefix . '-' . time() . '.css';
+    /** @var \Drupal\Core\File\FileUrlGeneratorInterface $file_url_generator */
+    $file_url_generator = \Drupal::service('file_url_generator');
+    // $css_file_directory_path = $file_url_generator->generateString(self::CSS_FILE_DIRECTORY);
+    $css_file_directory_path = \Drupal::service('file_system')->realpath(self::CSS_FILE_DIRECTORY);
+    $finder = new Finder();
+    $finder->name($prefix . '-*.css');
+    foreach ($finder->in($css_file_directory_path) as $file) {
+      unlink($file->getPathname());
+    }
+    
+    if (\Drupal::service('file_system')->saveData($content, self::CSS_FILE_DIRECTORY . '/' . $new_file_name, FileSystemInterface::EXISTS_REPLACE)) {
+      return $new_file_name;
+    }
+  }
+}
\ No newline at end of file
diff --git a/wysiwyg_template.libraries.yml b/wysiwyg_template.libraries.yml
new file mode 100644
index 0000000..f1501cd
--- /dev/null
+++ b/wysiwyg_template.libraries.yml
@@ -0,0 +1,4 @@
+view-template:
+  css:
+    theme:
+      view.css: { type: external }
\ No newline at end of file
diff --git a/wysiwyg_template.links.menu.yml b/wysiwyg_template.links.menu.yml
index 85e1dfc..4bfc512 100644
--- a/wysiwyg_template.links.menu.yml
+++ b/wysiwyg_template.links.menu.yml
@@ -5,3 +5,9 @@ wysiwyg_template.collection:
   description: 'Create and modify WYSIWYG templates'
   parent: system.admin_config_content
 
+wysiwyg_template.settings:
+  route_name: entity.wysiwyg_template.settings
+  title: 'Settings'
+  weight: 11
+  description: 'Adjust Wysiwyg Template settings'
+  parent: wysiwyg_template.collection
diff --git a/wysiwyg_template.links.task.yml b/wysiwyg_template.links.task.yml
new file mode 100644
index 0000000..cedc56f
--- /dev/null
+++ b/wysiwyg_template.links.task.yml
@@ -0,0 +1,11 @@
+wysiwyg_template.collection:
+  route_name: entity.wysiwyg_template.collection
+  title: 'WYSIWYG templates'
+  weight: 10
+  base_route: wysiwyg_template.collection
+
+wysiwyg_template.settings:
+  route_name: entity.wysiwyg_template.settings
+  title: 'Settings'
+  weight: 11
+  base_route: wysiwyg_template.collection
diff --git a/wysiwyg_template.module b/wysiwyg_template.module
index fbbe76d..5f5de25 100644
--- a/wysiwyg_template.module
+++ b/wysiwyg_template.module
@@ -5,6 +5,9 @@ use Drupal\Core\Form\FormStateInterface;
 use Drupal\node\NodeTypeInterface;
 use Drupal\node\Entity\NodeType;
 use Drupal\wysiwyg_template\Entity\Template;
+use Drupal\wysiwyg_template\Form\SettingsForm;
+use Symfony\Component\Finder\Finder;
+use Drupal\Core\File\FileSystemInterface;
 
 /**
  * Implements hook_editor_js_settings_alter().
@@ -87,3 +90,89 @@ function wysiwyg_template_form_node_form_alter(array &$form, FormStateInterface
     $form['body']['widget'][0]['#default_value'] = $template->getBody();
   }
 }
+
+/**
+ * Implements hook_ckeditor_css_alter()
+ */
+function wysiwyg_template_ckeditor_css_alter(array &$css, Drupal\editor\Entity\Editor $editor) {
+  /** @var \Drupal\Core\File\FileUrlGeneratorInterface $file_url_generator */
+  $file_url_generator = \Drupal::service('file_url_generator');
+
+  $editor_css_file = \Drupal::state()->get('wysiwyg_template.editor_css_file');
+
+  $regenerate_css_file = FALSE;
+
+  if (!empty($editor_css_file)) {
+    $regenerate_css_file = !file_exists(SettingsForm::CSS_FILE_DIRECTORY . '/' . $editor_css_file);
+  }
+  else {
+    $regenerate_css_file = TRUE;
+  }
+  if ($regenerate_css_file) {
+    $editor_css_file = wysiwyg_template_regenerate_css_files('editor_css');
+    \Drupal::state()->set('wysiwyg_template.editor_css_file', $editor_css_file);
+  }
+  $css[] = $file_url_generator->generateString(SettingsForm::CSS_FILE_DIRECTORY . '/' . $editor_css_file);
+}
+
+/**
+ * Implements hook_preprocess_node()
+ */
+function wysiwyg_template_preprocess_node(&$variables) {
+  if (isset($variables['elements']['body'])) {
+    $variables['#attached']['library'][] = 'wysiwyg_template/view-template';
+  }
+}
+
+
+/**
+ * Implements hook_library_info_alter()
+ */
+function wysiwyg_template_library_info_alter(&$libraries, $extension) {
+  if ($extension == 'wysiwyg_template' && isset($libraries['view-template'])) {
+    $libraries['view-template']['css']['theme'] = array(
+      file_create_url(SettingsForm::getViewCSSFilePath()) => array(
+        'type' => 'external',
+      ),
+    );
+  }
+}
+
+/**
+ * Utility function to call for regenerating CSS files.
+ */
+function wysiwyg_template_regenerate_css_files($type) {
+  $config = \Drupal::config('wysiwyg_template.settings');
+  if ($type == 'editor_css') {
+    $editor_css_file = SettingsForm::regenerateCSSFile('editor', $config->get($type));
+    return \Drupal::state()->set('wysiwyg_template.editor_css_file', $editor_css_file);
+  }
+  else if ($type == 'view_css') {
+    return \Drupal::service('file_system')->saveData($config->get($type), SettingsForm::getViewCSSFilePath(), FileSystemInterface::EXISTS_REPLACE);
+  }
+}
+
+
+/**
+ * Implements hook_cache_flush()
+ */
+function bwma_cache_flush() {
+  $dir = \Drupal::service('file_system')->realpath('public://mce/');
+
+  $finder = new Finder();
+  $finder->name('*.css');
+  $finder->files()->in($dir);
+
+  foreach ($finder as $file) {
+    $absoluteFilePath = $file->getRealPath();
+    unlink($absoluteFilePath);
+  }
+}
+
+/**
+ * Implements hook_rebuild()
+ */
+function bwma_rebuild() {
+  wysiwyg_template_regenerate_css_files('editor_css');
+  wysiwyg_template_regenerate_css_files('view_css');
+}
diff --git a/wysiwyg_template.routing.yml b/wysiwyg_template.routing.yml
index 85a0a4a..d5c508d 100644
--- a/wysiwyg_template.routing.yml
+++ b/wysiwyg_template.routing.yml
@@ -49,3 +49,11 @@ entity.wysiwyg_template.delete_form:
     _title: 'Delete template'
   requirements:
     _permission: 'administer wysiwyg templates'
+
+entity.wysiwyg_template.settings:
+  path: '/admin/config/content/wysiwyg-templates/settings'
+  defaults:
+    _form: '\Drupal\wysiwyg_template\Form\SettingsForm'
+    _title: 'Wysiwyg Template Settings'
+  requirements:
+    _permission: 'administer wysiwyg templates'