Adding JavaScript and CSS to Drupal 8

To add to a Drupal module:

https://www.drupal.org/node/2274843

To add to a Drupal theme:

https://www.drupal.org/node/2216195

create a hello_world.libraries.yml (module_name.libraries.yml) file and inside it would look like this:

create a hello_world.libraries.yml (module_name.libraries.yml) file and inside it would look like this:

hello-world:
  version: 1.x
  css:
    theme:
      css/hello_world.css: {}


Then you can attach the css within the library to the render array like so:
 $element = array(
      '#markup' => '
Saying Hello World in Drupal 8 is cool!
' . $content,
      '#attached' => array(
        'library' => array(
          'hello_world/hello-world',
        ),
      ),
    );
  return $element;