Remove Stable 9 as base theme

Enabling twig debug does show traces of stable9 being used when bootstrap is enabled.

Here are the traces I found.

Templates

<!-- BEGIN OUTPUT from 'core/themes/stable9/templates/block/block--system-messages-block.html.twig' -->

<!-- BEGIN OUTPUT from 'core/themes/stable9/templates/block/block--system-menu-block.html.twig' -->

Stylesheets

<link rel="stylesheet" media="all" href="/core/themes/stable9/css/system/components/align.module.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/system/components/container-inline.module.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/system/components/clearfix.module.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/system/components/hidden.module.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/system/components/js.module.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/core/components/progress.module.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/core/components/ajax-progress.module.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/toolbar/toolbar.module.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/contextual/contextual.module.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/contextual/contextual.toolbar.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/toolbar/toolbar.menu.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/toolbar/toolbar.theme.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/toolbar/toolbar.icons.theme.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/contextual/contextual.theme.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/contextual/contextual.icons.theme.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/user/user.icons.admin.css?thm2dh">
<link rel="stylesheet" media="all" href="/core/themes/stable9/css/core/assets/vendor/normalize-css/normalize.css?thm2dh">
<link rel="stylesheet" href="/core/themes/stable9/css/views/views.module.css?thm2dh" media="all">

Attempted an MR.

So, add it as a patch in composer.json:

            "drupal/bootstrap5": {
                "#3608124 - remove stable9 theme": "https://git.drupalcode.org/project/bootstrap5/-/merge_requests/115.diff"
            }

Then run:

drush cr
drush updb -y

And check if everything still works for you.

For me everything is fine. It removed the warning at /admin/reports/status about deprecated stable9 theme, and my custom theme based on bootstrap5 still works. The MR was created on bootstrap5 4.0.x-dev but it also applies to bootstrap5 4.0.8

As a short term fix, there is a contrib stable9 that works.

https://www.drupal.org/project/stable9

I helped fix a bunch of CSS issues and got rid of the deprecation error specifically for my bootstrap5-themed site.

Seems like it is working well now.

The patch from #5 worked for me. The warning is gone from the Drupal status page and the site seems to be working normally.

The twig templates which were previously being pulled from stable9 are now coming from system.

<!-- BEGIN OUTPUT from 'core/modules/system/templates/block--system-messages-block.html.twig' -->
<!-- BEGIN OUTPUT from 'core/modules/system/templates/block--system-menu-block.html.twig' -->

And, as expected, all references to stable9 css are gone as well.

From the two possible ways of going forward:
- make bootstrap5 theme depend on contrib stable9 theme / keep the bootstrap5 theme having stable9 as base theme
- make bootstrap5 theme independant / set base_theme: false

I prefer this MR115 and not having to depend on a whole other contrib theme.

Myself and mr.white both reported everything working without depending on stable9 theme so its at least partly tested by the community.
I wouldn't mind getting more opinions on this to confirm that its a good idea to let this bootstrap5 theme not have a base theme.

I ran into this exact same issue with D11.4. I applied the patch, ran the database update and no longer see the error message. I also agree that removing the dependency of Stable9 is the way to go, and I believe that was the intention of the latest Bootstrap5 version as well.

Thanks for reporting and working on this. Since more and more users are upgrading to Drupal 11 and running into this, should the priority be raised?

I have updated the Issue Summary and removed the mentions of uninstalling stable9 with Drush, since it is a Bootstrap5 base theme, and might corrupt an installation.

Since many users report that the MR works, perhaps one of you reviewers can change the Status to RTBC?
(The tests seem to be failing generally, and could be a false alarm.)