.info.yml is Drupal cached file.

With using new theme just created in above step, we must clear the cache.

Some option to clear the cache on Drupal 8.

Use Drupal UI : navigating to Configuration > Performance (admin/config/development/performance) page and clicking the 'Clear all caches' button.

Use Drupal console or drush (version >=8 ) : run command drush cr or drupal cr all

Finally, navigate to the Appearance (admin/appearance) page. You should see your Ice Cream theme listed under Uninstalled themes

Click Install or Install and set as default