Embed external fonts in Drupal 8 theme (fonts are added in custom themename.libraries.yml)

themename.info.yml

name: Theme name
type: theme
description: Theme description
package: Custom
core: 8.x
libraries:
  - themename/fonts
  - themename/global-styling

base theme: classy

# Regions
regions:
  header: 'Header'
  primary_menu: 'Navigation'
  highlighted: 'Preface'
  content: 'Content'
  footer: 'Footer'

themename.libraries.yml

fonts:
  license:
    name: Apache License, 2.0
    url: http://www.apache.org/licenses/LICENSE-2.0.html
  css:
    theme:
      '//fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800': { type: external }

global-styling:
  version: 1.x
  css:
    theme:
      css/bootstrap/bootstrap.min.css: {}
      css/custom.css: {}
  js:
    js/bootstrap.min.js: {}
    js/custom.js: {}
  dependencies:
    - core/jquery
    - core/jquery.once