Theme — purely visual styling (“look-and-feel”) for a component.

Purely visual styling, such as border, box-shadow, colors and backgrounds,
font properties, etc. Ideally, these should be separated enough from a 
component’s structure to be “swappable”, and omitting these entirely should
not break the component’s functionality or basic usability.

https://drupal.org/docs/develop/standards/css/css-file-organization-for-drupal-8
