The asset management system is the most recent one to Drupal 8. The asset management system allows modules and themes to register libraries. Libraries define CSS style sheets and JavaScript files that need to be loaded with the page. Drupal 8 takes this approach for the frontend performance. Rather than loading all CSS or JavaScript assets, only those required for the current page in the specified libraries will be loaded.
In this recipe, we will define a libraries.yml file that will define a CSS style sheet and JavaScript file provided by a custom theme.