Add print.css to theme in Drupal 8

Edit THEME.libraries.yml. Add the highlighted line:

global-styling:
  css:
    theme:
      css/glazed.css: { minified: true }
      css/print.css: { media: print }
  js:
    vendor/classie.js: {}
    js/minified/glazed-mobile-nav.min.js: {}
    js/minified/glazed.min.js: {}
  dependencies:
    - core/jquery

   [...]

Create the print.css file at the indicated location (.e.g., css/print.css).

Add css rules to the print.css file.

Clear the cache and review changes by forcing Chrome into print emulation.