With libraries, you can specify CSS by different groups. Drupal's asset management system provides the following CSS groups:
- Base
- Layout
- Component
- State
- Theme
Style sheets are loaded in the order in which the groups are listed. Each one of them relates to a PHP constant defined in /core/includes/common.inc. This allows separation of concerns when working with style sheets. Drupal 8's CSS architecture borrows concepts from the Scalable and Modular Architecture for CSS (SMACSS) system to organize CSS declarations. You can learn more about this technique for building flexible and scalable CSS style sheets at https://smacss.com/.