Child theme loses wp-admin styling

I found that adding this to my ‘smart-mag-child’ functions.php file worked.

add_editor_style( get_template_directory_uri() . '/style.css' );

If someone can show me documentation to how to properly do this that would be appreciated.

Edit:
Dokumentation about this to understanding why it works.

• add_editor_style() – Add callback for custom TinyMCE editor stylesheets.
• get_template_directory_uri() – Retrieve theme directory URI. In the case a child theme is being used, the absolute path to the parent theme directory will be returned. Use get_stylesheet_directory_uri() to get the absolute path to the child theme directory.