Drupal 9 Sutheme for Bartik Core Theme

Bartik is the default core theme for Drupal since D7. Earlier D8 Bartik subtheme does not work on new D9; the subtheme configuration has changed. The task is to provide a standard Bartik subtheme that works in D9, mainly for the website operator to perform simple css modifications. Edit: clarified "...provide a standard _Bartik_ subtheme that works in D9... "