Change record status: 
Project: 
Introduced in branch: 
8.0.x
Introduced in version: 
8.0.0-RC1
Description: 

Libraries that have been defined in *.libraries.yml can now be overridden or extended using the libraries-override and libraries-extend entries in a theme's *.info.yml file.

Libraries-override

libraries-override was added as a way for themes to manipulate assets (CSS/JS) provided by libraries. This includes removing or replacing assets, as well as removing a library altogether.

libraries-override:
  # Replace an entire library.
  core/drupal.collapse: mytheme/collapse
  
  # Replace an asset with another.
  subtheme/library:
    css:
      theme:
        css/layout.css: css/my-layout.css
  
  # Remove an asset.
  drupal/dialog:
    css:
      theme:
        dialog.theme.css: false
  
  # Remove an entire library.
  core/modernizr: false

Libraries-extend

libraries-extend provides a way for themes to alter the assets of a library by adding in additional theme-dependent library assets whenever a library is attached.
Libraries-extend are specified by extending a library with any number of other libraries

# Extend drupal.user: from core.libraries.yml and add assets to 
# classy's(classy.libraries.yml) user1 & user2 libraries .
libraries-extend:
  core/drupal.user: 
    - classy/user1
    - classy/user2

Sub-themes inherit the libraries-override and libraries-extend specified in base themes.

Examples of libraries-override and libraries-extend can be found in Bartik and Seven themes.

Impacts: 
Themers
Updates Done (doc team, etc.)
Online documentation: 
Not done
Theming guide: 
Not done
Module developer documentation: 
Not done
Examples project: 
Not done
Coder Review: 
Not done
Coder Upgrade: 
Not done
Other: 
Other updates done