Example Drupal 8 info file for removing base theme CSS or Libraries

theme_info.yml

# Extend an existing library:
# Ensures overrides are only added when the toolbar is available.
libraries-extend:
  toolbar/toolbar.menu:
    - my_theme/drupal
    
    
# Replace or remove entire libraries;
# Un-comment each library to exclude it's CSS, JS and Dependencies from being loaded.

libraries-override:
  core/normalize: false
  core/html5shiv: false
  system/base: false
  core/jquery.ui:
    css:
      theme:
        assets/vendor/jquery.ui/themes/base/theme.css: false
  
# CKEDITOR
  
ckeditor_stylesheets:
  - css/ckeditor.css

ckeditor5-stylesheets:
  - css/ckeditor5.css