How can I debug adding libraries via hooks?
To attach a library to a certain paragraph type you could use template_preprocess_paragraph__PARAGRAPH_TYPE() like following. Note how it's just added to the $variables array not the element.

function MYTHEME_preprocess_paragraph__slider(&amp;$variables) {
  $variables['#attached']['library'][] = 'MYTHEME/carousel-styling';
}


Same with the field. Simply use template_preprocess_field__FIELD_NAME() and attach the library directly to the $variables array.



Your content-styling and carousel-styling libraries probably should look more like following. Note how it's added as theme category, not as base. See the docs on File Structure for some info about SMACSS.

content-styling:
  css:
    theme:
      css/content.css: {}




And finally: Flush cache, flush cache and flush the cache again!!!!!!!