breakpoints.yml

yourtheme.mobile:
  label: mobile
  mediaQuery: '(min-width: 0px)'
  weight: 2
  multipliers:
    - 1x
yourtheme.narrow:
  label: narrow
  mediaQuery: 'all and (min-width: 560px) and (max-width: 860px)'
  weight: 1
  multipliers:
    - 1x
yourtheme.wide:
  label: wide
  mediaQuery: 'all and (min-width: 851px)'
  weight: 0
  multipliers:
    - 1x

global-styling.yml

global-styling:
  css:
    theme: 
      css/styles.css: {}
      css/layout.css: {}
      css/print.css: { media: print }

  js:
    js/yourtheme.js: {}
    js/custom-library.js: {}

  dependencies:
    - core/jquery
    - core/drupal

custom-stuff:
  css:
    theme:
      css/print.css: {}
      css/another-style.css: {}

  js:
    js/yourtheme.js: {}
    js/haha.js: {}

info.yml

name: YourTheme
description: 'A Cool theme by you.'
type: theme
package: Custom
base theme: classy
core: 8.x
screenshot: yourtheme.png
libraries:
  - yourtheme/global-styling
regions:
  header: ‘Header’
  primary_menu: 'Primary menu'
  secondary_menu: 'Secondary menu'
  help: Help
  page_top: 'Page top'
  page_bottom: 'Page bottom'
  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’

libraries-css.yml

css:
  theme: 
    css/styles.css: {}
    css/layout.css: {}
    css/print.css: { media: print }

libraries-js.yml

js:
  js/yourtheme.js: {}
  js/custom-library.js: {}

libraries.yml

libraries:
  - yourtheme/global-styling

libraries2.yml

libraries:
  - yourtheme/global-styling
  - yourtheme/custom-stuff

regions.yml

regions:
  header: ‘Header’
  primary_menu: 'Primary menu'
  secondary_menu: 'Secondary menu'
  help: Help
  page_top: 'Page top'
  page_bottom: 'Page bottom'
  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’