How to remove a system CSS file in a Drupal 8 theme?
<p>I am trying to remove Drupal CSS files (such as ajax-progress.module.css) from my custom theme YML. Here is what I tried:</p>

<pre><code>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
</code></pre>

<p>Drupal's documentation about that is scarce at best...</p>