Why is the CSS for my theme not loading while the JS is? Theme mytheme is placed under /sites/all/themes/

Content of mytheme.info.yml :

name: My Theme
type: theme
description: Custom theme.
core: 8.x
base theme: classy
version: 8.x-1.0
libraries:
  - mytheme/global-styling
regions:
  logo: 'Logo'
  navigation: 'Navigation'
  page_header: 'Page Header'
  content: Content  # the content region is required
  page_sidebar: 'Page Sidebar'
  contact_first: 'Contact (First column)'
  contact_second: 'Contact (Second column)'
  contact_third: 'Contact (Third column)'
  footer_logo: 'Footer logo'
  footer_first: 'Footer first'
  footer_second: 'Footer second'
  footer_third: 'Footer third'
  footer_copyright: 'Footer Copyright'
regions_hidden:
  - sidebar_first


Content of mytheme.libraries.yml :

global-styling:
  version: 1.x
  css:
    theme:
      assets/plugins/bootstrap/css/bootstrap.min.css: { minified: true }
      assets/plugins/font-awesome/css/font-awesome.min.css: { minified: true }
      assets/css/style.css: {}
      assets/fonts/montserrat/montserrat.css: {}
  js:
    assets/plugins/jquery-1.11.1.min.js: { minified: true }
    assets/plugins/bootstrap/js/bootstrap.min.js: { minified: true }
    assets/plugins/moderniz.js: {}
    assets/js/app.js: {}
  dependencies:
    - core/jquery
    - core/drupal
    - core/drupalSettings


The JS is loading but the CSS isn't. How come?
try wrapping the google fonts entry in quotes like this:

'//fonts.googleapis.com/css?
family=Open+Sans:400,300,600&amp;subset;=latin,cyrillic': { type: external }


Maybe there is a problem with the url using yaml special characters https://stackoverflow.com/questions/19109912/do-i-need-quotes-for-strings-in-yaml#22235064