The CERN Theme is the recommended Drupal theme because it defines a common Look & Feel for the CERN websites. In other words, the websites that use the CERN Theme share specific common characteristics which relate to the styling, the appearance and specific features which are supported by the CERN Theme.
In addition, the theme inherits the Drupal Bootstrap theme, which consequently means that it includes all the elements of the Bootstrap theme.
How to enable the CERN theme
The CERN Theme is placed in the CERN infrastructure, meaning that you don't have to actually install the theme in the website. When you get a new CERN website, the theme is already installed and enabled. If for any reason the theme is not enabled, you can enable by clicking on "Set as default" option in the Appearance page.