Drupal 8: Removing Core Stylesheets in a Sub-theme

With a sub-theme, sometimes you need to remove some base-theme's stylesheets so it's easier to theme within your sub-theme. In that case, we can use the libraries-override function, new in Drupal 8. You'd add this to your theme's theme.info.yml file. In the example below, we are using core's classy theme as a base-theme.

# Basic theme info.
name: Dolphinea
desciption: A Drupal 8 theme
type: theme
base theme: classy
core: 8.x
version: VERSION
screenshot: screenshot.png

# Remove unneeded stylesheets.
libraries-override:
  classy/base:
    css:
      component:
        css/components/textarea.css: false
        css/components/menu.css: false
        css/components/item-list.css: false
        css/components/links.css: false
        css/components/form.css: false
        css/components/pager.css: false

Of note here is classy/base > css > components. That follows the theme path from within Classy, i.e. /core/themes/classy/css/components.