How can I access individual form elements for theming?
That's a case for the new Formdazzle! module.


  Drupal form theming with less pain
  
  Theming drupal forms can be difficult and time-consuming. This module
  provides a set of utilities that make form theming easier.
  
  Currently, this module provides theme suggestions for forms that are
  much more useful than those provided by Drupal core.
  
  
  Theme suggestions for all form elements (including buttons)
  Theme suggestions for all form element labels
  All theme suggestions include the form ID and the form element name; e.g. [element-type]--[form-id]--[form-element-name].html.twig
  
  
  While Drupal core only provided these two theme suggestions:

input.html.twig
input--textfield.html.twig

  
  Formdazzle adds the following two theme suggestions to the list:

input--textfield--webform-contact-form.html.twig
input--textfield--webform-contact-form--first-name.html.twig