New experimental administration theme: Claro!

Drupal's admin UI (the Seven theme) feels outdated and needs a visual update. The current designs were created for Drupal 7 in 2008-2009 and were slightly updated for Drupal 8. At the time, many of the current Drupal's features and components didn't exist and Seven's UX was not taking them into account.

This has been raised by agency leaders as one of the key issues preventing them from selling Drupal. A Drupal admin UI UX study that was done in 2018 also validated the need for a more modern admin UI.

Claro was validated in contrib and has thousands of sites using it, and several Drupal distributions have already adopted it. Claro has been implemented using the new Drupal Design System.

Installation instructions for sites using Seven.