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...

For others that have spent countless hours getting this to work and are struggling... in particular assigning the correct user to the comment - this is how I achieved the correct outcome. The outline below is for usage via Postman and DHC (Restlec Client)... once working here then testing done and ready to apply to your app or however you are using this. Set up Set accept to hal+json sample: POST request url: http://example.com/entity/comment?_format=hal_json Authorization Type: Basic {then...