Uninstalled custom theme stuck in config blocking config import

For the change record mentioned in #40.

Did not test patch.

Added a basic change record. Keeping this on needs work for the documentation.

Fixed remarks in #40

CR has been added, in addition to the added comments.

Manually ran the tests and all green.

Could this of been used between 9 and 10 to uninstall classy?

+++ b/core/lib/Drupal/Core/Extension/ThemeInstaller.php
@@ -266,9 +266,6 @@ public function uninstall(array $theme_list) {
-      if (!isset($list[$key])) {
-        throw new UnknownExtensionException("Unknown theme: $key.");
-      }

I think we should replace this with

      if ($extension_config->get("theme.$key") === NULL) {
        throw new UnknownExtensionException("Unknown theme: $key.");
      }

So we can only uninstall themes that are listed in the core.extension.

Otherwise this happens:

vendor/bin/drush thun blah                                                                                                                                                           
 [warning] The following theme is missing from the file system: blah ExtensionPathResolver.php:63
 [success] Successfully uninstalled theme: blah

Looks like the test coverage changes were also wrong, updated test + code based on #47.

Manually triggering tests and hiding old patches.

All green. And feedback from #47 has been addressed.

Committed and pushed bfc5c6d181c to 11.x and 01b19986028 to 10.2.x. Thanks!

Backported to 10.2.x as a non-disruptive bugfix.

Automatically closed - issue fixed for 2 weeks with no activity.