Drupal CMS Olivero
This theme provides a clean, out-of-the-box presentation for core content types in Drupal CMS versions 1.x. It is not intended for use in Drupal CMS 2.x and later, and customization requires forking the theme rather than subtheming.
Included in Drupal CMS 1.x
This theme is part of Drupal CMS 1.x. It is intended to make the content types that ship with that version of Drupal CMS look presentable out of the box:
Not included in Drupal CMS 2.x anymore
Drupal CMS 2.x and later will use site templates and Drupal Canvas to create their look and feel, and therefore won't use this theme. It is minimally maintained in order to support sites that were built on Drupal CMS 1.x, but it will receive no new features and won't support Canvas.
Unless you are running a site built on Drupal CMS 1.x, using this theme is not recommended.
Customization
If you need to customize this theme, don't subtheme it! This theme is minimally maintained and everything in it is considered internal. To customize this theme, you should fork it: copy it into your site's themes/custom directory, rename it (including the directory itself, the info file, theme hooks, and so forth), and then commit it to your repository.