Change record status: 
Project: 
Introduced in branch: 
8.8.x
Introduced in version: 
8.8.0-alpha1
Description: 

Ajax progress bar wrapper can now be customized by overriding Drupal.theme.ajaxProgressBar theme function.

The default implementation looks following:

Drupal.theme.ajaxProgressBar = $element => $('<div class="ajax-progress ajax-progress-bar"></div>').append($element);
Impacts: 
Themers