Thanks to Drupal Rector, a tool developed by Palantir.net and Pronovix, most of the Drupal 9 modules and themes can be automatically fixed to adapt it for Drupal 10. The tool can fix many of the deprecated API uses needed to make a module Drupal 10 compatible.
The fourth feature release of Drupal 9 brings a whole new frontend look with the Olivero theme by default and a refreshed backend interface with the Claro theme. Claro had became default backend theme from the major version release of Drupal 9.0.0 itself and is now stable. There is also a new starterkit theme generator, better image loading performance and easier permission management.