Issue #3116999 by manuel.adan, Sahana _N: Drupal 9 compatibility

custom_markup_block.info.yml

@@ -2,6 +2,7 @@ name: Custom Markup Block
 type: module
 description: A custom markup block allows adding any markup to a block which gets saved to config.
 core: 8.x
+core_version_requirement: ^8 || ^9
 package: Other
 version: 8.x-1.0
 dependencies:

tests/src/Functional/CustomMarkupBlockTestBase.php

@@ -9,6 +9,11 @@ use Drupal\Tests\block\Functional\BlockTestBase;
  */
 abstract class CustomMarkupBlockTestBase extends BlockTestBase {
 
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
   /**
    * Modules to enable.
    *
@@ -18,4 +23,15 @@ abstract class CustomMarkupBlockTestBase extends BlockTestBase {
     'custom_markup_block',
   ];
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    if (version_compare(\Drupal::VERSION, '9', '<')) {
+      $this->defaultTheme = 'classy';
+    }
+
+    parent::setUp();
+  }
+
 }