How can I style the WYSIWYG content?
You can either implement hook_ckeditor_css_alter or you can add ckeditor_stylesheets to your themes info.yml file.

See: hook_ckeditor_css_alter for more.

Example:

function mymodule_ckeditor_css_alter(array &amp;$css, Editor $editor) {
  $css[] = drupal_get_path('theme', 'mytheme') . '/css/mymodule-ckeditor.css';
}


Or in mytheme.info.yml:

ckeditor_stylesheets:
  - css/styles.css