In a recent update made by Drupal 8, they have changed the way the asset libraries for both JS and CSS are attached to the theme. You will need to think of adopting these strategies for a variety of scenarios. Here are some of the widely thought of strategies that you can use to add JS and CSS libraries to your Drupal 8 application or website.
Global Styles and Scripts
If you want to add the CSS or JS assets through global libraries then here's how you can possibly achieve that.
The name of the global library is global-styling which will help in creating global styles, and it has been declared within the theme_name.libraries.yml. It is also added within the theme_name.info.yml. You can avail this library through the website, as it has been declared as the global library. The theme_name.libraries.yml file will appear as follows