Drupal 8 Quick Bartik subtheme

mybartiksub.info.yml

name: My Bartik subtheme
type: theme
core: 8.x
base theme: bartik
description: 'Bartik subtheme.'
alt text: 'Bartik subtheme.'

libraries:
  - 'mybartiksub/theme'

# These are copied from bartik.info.yml
regions:
  header: Header
  primary_menu: 'Primary menu'
  secondary_menu: 'Secondary menu'
  page_top: 'Page top'
  page_bottom: 'Page bottom'
  highlighted: Highlighted
  featured_top: 'Featured top'
  breadcrumb: Breadcrumb
  content: Content
  sidebar_first: 'Sidebar first'
  sidebar_second: 'Sidebar second'
  featured_bottom_first: 'Featured bottom first'
  featured_bottom_second: 'Featured bottom second'
  featured_bottom_third: 'Featured bottom third'
  footer_first: 'Footer first'
  footer_second: 'Footer second'
  footer_third: 'Footer third'
  footer_fourth: 'Footer fourth'
  footer_fifth: 'Footer fifth'

mybartiksub.libraries.yml

theme:
  css:
    theme:
      'site.css': { }

site.css

/* Custom CSS goes here. */