Unable to remove the theme.

Hi.

The root of the issue is material_stable9 is a hidden theme, so it can't be uninstalled via appearance page, usually.

Could you please try this:

1. composer require 'drupal/material_base:^3.0'
2. drush cr
3. drush thun material_base_mdc
4. drush thun material_base
5. drush thun material_stable9
6. drush cr
7. drush cron
8. composer removee drupal/material_base

Hope it helps.

Thank you. No joy.

composer require 'drupal/material_base:^3.0'
./composer.json has been updated
Running composer update drupal/material_base
Loading composer repositories with package information
Updating dependencies
Lock file operations: 1 install, 0 updates, 0 removals
  - Locking drupal/material_base (3.0.0)
Writing lock file
Installing dependencies from lock file (including require-dev)
Package operations: 1 install, 0 updates, 0 removals
  - Installing drupal/material_base (3.0.0): Extracting archive
Generating autoload files
Hardening vendor directory with .htaccess and web.config files.
57 packages you are using are looking for funding.
Use the `composer fund` command to find out more!
Cleaning installed packages.
No security vulnerability advisories found.

Then

drush cr
 [success] Cache rebuild complete.

So far, so good. Moving on...

drush thun material_base_mdc

In ThemeInstaller.php line 263:
                                     
  Unknown theme: material_base_mdc.  

I also tried #4, just to see what happened. Same result. I saw no point in attempting to continue.

There is a point to continue. I added #3 and #4 to be 100% sure nothing is blocking #5 and the rest of the commands.
The main commands are #1, #5, and #7.

Yes, you're right. I was concerned as #3 and #4 both came back with what appeared to be errors, but #5 worked fine, and completing the whole process the themes were uninstalled, and the update routine ran without issue.

Thank you, very much. I appreciate your help and your patience.