Deprecated core themes cannot be disabled on the "Appearance" tab because they are hidden

@Spokje, I just realized that I never acknowledged your comment #13. No offense taken, but yes, I did clear caches with no effect. I also tried installing Bootstrap5, then removing it, but Classy and Stable still have no uninstall options.

@trolin: I had the same issue, and discovered when I tried to uninstall via Drush that one of my installed themes has Stable as its base_theme—maybe that's why you don't see the uninstall option after unhiding?

@noah, you win the prize. I'm using theme danland which depends on classy which depends on stable. Thanks for the tip.

Unlike D7, D9 seems to have a dearth of attractive themes ready for use. I hope that situation improves. I'll keep looking.

workaround #6 worked for me.

I also didn't see an uninstall button on Stable noted in #11. Once i clicked uninstall on Classy, the Stable panel shifted and now there was an uninstall button there.

Another way to achieve this, if you are using the config exporter, is to edit your config/core.extension.yml file and remove the "classy" and "stable" elements under "theme"

Just reiterating #6 worked for me, if you simply need to get this done now.

Confirming that #24 worked for me - do a 'drush cim' after the edit.

In my case I cannot uninstall Classy and Stable, because my custom theme (News+ Lite) is based on Classy .

Time to update my custom theme to a new Drupal9 base theme.

Updated the patch for this and added a test. Happy for opinions on if this is the right approach & the wording of the status message.

The last submitted patch, 28: 3328016-28-test-only.patch, failed testing. View results →

Fixing CCF from my earlier patch. Tests only patch in #28 is unchanged.

The last submitted patch, 30: 3328016-30.patch, failed testing. View results →

I'm on 9.5.10 and prepping to upgrade to Drupal 10 - I see no uninstall option as shown in #12. I could drush pm-uninstall, I suppose, but I'm using a subtheme of Bootstrap5 2.0.9. So if the following from #18 holds true for 2.0.9...

You need to uninstall Bootstrap5 version 2.0.5 if you want to be able to uninstall Classy and Stable. The reason for this is that Bootstrap5 2.0.5 is set up with Classy as a base theme, and it in turn depends on Stable.

I could end up completely borking the site. Any ideas? Suggestions greatly appreciated!

To prepare for Drupal 10, upgrade from Bootstrap5 version 2.0.5 to version 3.0.9. Then the uninstall link should appear.

Thanks for the quick response!

I installed Bootstrap5 3.0.9 and it seemed to work initially - cleared caches and crashed.

Reading watchdog via Drush I got the following:

Drupal\Core\Theme\MissingThemeDependencyException:
Base theme stable9 has not been installed.
in Drupal\Core\Theme\ThemeInitialization->getActiveThemeByName() (line 115 of /[site]

Saw this post:

https://designkojo.com/base-theme-stable9-has-not-been-installed

Enabled stable9 using Drush:

drush theme-enable stable9

Site came back up and seems to be working, but I get a DB upgrade warning.

Trying to update DB results in the following:

[site]# drush updb
 ------------ ------------------------- ------------- ----------------------- 
  Module       Update ID                 Type          Description            
 ------------ ------------------------- ------------- ----------------------- 
  bootstrap5   install_stable9           post-update   Enable new theme:      
                                                       stable9.               
  bootstrap5   uninstall_stable_classy   post-update   Uninstall old themes:  
                                                       stable and classy.     
 ------------ ------------------------- ------------- ----------------------- 


 Do you wish to run the specified pending updates? (yes/no) [yes]:
 > yes

>  [warning] Post update function bootstrap5_post_update_install_stable9 not found in file bootstrap5.post_update.php
>  [error]  Update failed: bootstrap5_post_update_install_stable9 
 [error]  Update aborted by: bootstrap5_post_update_install_stable9 
 [error]  Finished performing updates. 

At least the site seems to be working - any ideas here? Jeez...

Thanks for your help!

Update: Commenting out hidden: true in stable9.info.yml eliminates the DB error - let's see if I can make the transition to Drupal 10 and get rid of the deprecated themes there.

Patch #30 works for me, thank you!

As has been mentioned above, if a theme depends on Classy or Stable, then the uninstall for those will not appear, by design.

To find out if you have an unexpected dependency, run the drush uninstall command:

drush theme-uninstall classy

drush theme-uninstall stable

For me, those told me I had a dependency in my main site theme that needed to be looked at.

The workaround needs to be detailed on this CR https://www.drupal.org/node/3305674 → as otherwise people upgrading from 9 to 10 with Classy or Stable installed can't remove them.

To find out if you have an unexpected dependency, run the drush uninstall command:

drush theme-uninstall classy

drush theme-uninstall stable

Thanks, worked for me.
It happens that SimpleBlog theme is dependent on Classy theme, which is strange - because SimpleBlog is compatible with D10.

I summarized the workarounds in the Issue Summary