If you are using theme_get_settings, such as: $value = trim(theme_get_setting('my_setting')); Then in your code editor you will also be seeing undefined  function 'theme_get_setting'.  This function has been deprecated for some time.   Instead, you need to use Passing the theme.manager and theme.initialization services to theme_handler constructor as optional parameters is deprecated. These parameters are also mandatory. $theme_handler = \Drupal::service('theme_handler');Get...

In Drupal 8.x hiding the subject line is a quick task. In the admin area of your site go to structure > comment types > {comment type} > manage form display and the URL will be like https://domain.com/admin/structure/comment/manage/{comment}/form-display.  I have put the type in braces as you might have several comment types.  If you only have the default Drupal setting, then use default comments and the path will...