How do I remove a system CSS file in a theme? I am trying to remove Drupal CSS files (such as ajax-progress.module.css) from my custom theme YML. Here is what I tried:

name: Mercante
type: theme
description: Tema para o site Mercante.co
core: 8.x
stylesheets-remove:
  - core/themes/stable/css/system/components/ajax-progress.module.css
libraries-override:
  system/base:
    css:
      theme:
        css/components/ajax-progress.module.css: false


Drupal's documentation about that is scarce at best...

Update: another version that also does not work:

name: Mercante
type: theme
description: Tema para o site Mercante.co
core: 8.x
stylesheets-remove:
  - @stable/css/system/components/ajax-progress.module.css
libraries-override:
  system/base:
    css:
      component:
        /core/themes/stable/css/system/components/ajax-progress.module.css: false
        /core/themes/stable/css/system/components/autocomplete-loading.module.css: false
You can try with the unset method that drupal provides for css alter hook. Try the following code in your custom module 

function hook_css_alter(&amp;$css) { 
unset($css[drupal_get_path('module','system').'/system.theme.css']);}