Themes now use asset libraries exclusively, not individual stylesheets or JavaScript files

For consistency between modules and themes we are standardizing all asset attachment through libraries instead of including individual stylesheets and JavaScript files (that ability already was removed >6 months ago, see https://www.drupal.org/node/2228783) in a theme's THEME.info.yml file. The assets will be defined into a named library in a theme's THEME.libraries.yml and attached as such to the THEME.info.yml for loading assets on all pages, or through the #attached property of a renderable array or preprocess hook for assets that are only needed on some pages.

Before

example.info.yml:

name: Example
type: theme
core: 8.x
stylesheets:
  all:
    - css/style.css
  print:
    - css/print.css

After

example.libraries.yml:

libraryname:
  css:
    theme:
      css/style.css: {}
      css/print.css: { media: print }

example.info.yml

name: Example
type: theme
core: 8.x
libraries:
  - example/libraryname

Optional: Themes with more granular libraries

If you then e.g. want to load CSS to style nodes differently, but only want to load that CSS when actually necessary, you can expand example.libraries.yml to:

libraryname:
  css:
    theme:
      css/style.css: {}
      css/print.css: { media: print }

node:
  css:
    theme:
      css/node.css: {}

and in example.theme:

function example_preprocess_node(&$variables) {
  $variables['#attached']['library'][] = 'example/node';
}

This encourages logical groupings of assets to aid in correct order of dependencies (and note that it also allows you to put the associated JS in there as well), e.g.:

libraryname:
  css:
    theme:
      css/style.css: {}
      css/print.css: { media: print }

node:
  css:
    theme:
      css/node.css: {}

node-article:
  js:
    js/reading-time-indicator.js: {}
  css:
    theme:
      css/article.css: {}
  dependencies:
    - example/node

node-blog:
  css:
    theme:
      css/blog.css: {}
  dependencies:
    - example/node

Optional: SMACCS categorization

Note: the theme key in those YML files refers to the SMACSS structure used by Drupal 8, see https://www.drupal.org/node/1887922. If you don't care about SMACSS, use theme. If you do care about SMACSS, you can split up the style.css in the above example into more granular files, per SMACCS category:

libraryname:
  css:
    layout:
      css/grid.css: {}
    component:
      css/buttons.css: {}
      css/lists.css: {}
      css/pager.css: {}
      css/tables.css: {}
      css/tabs.css: {}
    theme:
      css/style.css: {}
      css/print.css: { media: print }

Optional: library-per-SMACCS-category

You could go even further, and create a library per SMACSS category for easier overriding in subthemes:

example.libraries.yml:

layout:
  css:
    layout:
      css/grid.css: {}

component:
  css:
    component:
      css/buttons.css: {}
      css/lists.css: {}
      css/pager.css: {}
      css/tables.css: {}
      css/tabs.css: {}

theme:
  css:
    theme:
      css/style.css: {}
      css/print.css: { media: print }

example.info.yml

name: Example
type: theme
core: 8.x
libraries:
  - example/layout
  - example/component
  - example/theme

For comprehensive docs, see https://www.drupal.org/theme-guide/8/assets