Install a theme for Drupal 8

Let’s pretend you want to install the Showcase lite theme for Drupal 8.

https://www.drupal.org/project/showcase_lite

Change directory to where composer.json resides

Ex.

cd /var/www/drupal-8

And run

composer require drupal/showcase_lite

This will place showcase_lite in /var/www/drupal-8/web/themes/contrib

Drupal 8 modules display the installation composer command which is particulary helpful when you want a different version of the project