For many years I have been using Drupal and develop a few module for the different versions over the years.

The site is now upgraded to Drupal 9.

Most of the module are possible to run with Drupal 9. The major issue was the theme used. It did work but was defined as a sub-theme of Bartik only compatible with Drupal 8.


name: Thuben 8
type: theme
description: This is the Thuben 8 sub theme of Bartik
# core: 8.x
core_version_requirement: 9.x
# Defines the base theme
base theme: bartik


With the above change it started working fine.