diff --git a/core/lib/Drupal/Core/Extension/ThemeHandler.php b/core/lib/Drupal/Core/Extension/ThemeHandler.php
index 1988a14..a17af76 100644
--- a/core/lib/Drupal/Core/Extension/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -483,4 +483,17 @@ public function getTheme($name) {
     throw new \InvalidArgumentException(sprintf('The theme %s does not exist.', $name));
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function hasUi($name) {
+    $themes = $this->listInfo();
+    if (isset($themes[$name])) {
+      if (!empty($themes[$name]->info['hidden'])) {
+        return $name == $this->getDefault();
+      }
+      return TRUE;
+    }
+    return FALSE;
+  }
 }
diff --git a/core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php b/core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php
index c46f96a..cb93e40 100644
--- a/core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php
@@ -208,4 +208,18 @@ public function themeExists($theme);
    */
   public function getTheme($name);
 
+  /**
+   * Determines if a theme should be shown in the user interface.
+   *
+   * To be shown in the UI the theme has to be installed. If the theme is hidden
+   * it will not be shown unless it is the default theme.
+   *
+   * @param string $name
+   *   The name of the theme to check.
+   *
+   * @return bool
+   *   TRUE if the theme should be shown in the UI, FALSE if not.
+   */
+  public function hasUi($name);
+
 }
diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index 29c6e16..f0bbdd3 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -88,8 +88,13 @@ function block_page_top(array &$page_top) {
  *   An array of theme names.
  */
 function block_themes_installed($theme_list) {
+  $theme_info = \Drupal::service('theme_handler')->listInfo();
   foreach ($theme_list as $theme) {
-    block_theme_initialize($theme);
+    // Don't initialize hidden themes as they are not displayed in the block
+    // management screens.
+    if (empty($theme_info[$theme]->info['hidden'])) {
+      block_theme_initialize($theme);
+    }
   }
 }
 
diff --git a/core/modules/block/src/Controller/BlockController.php b/core/modules/block/src/Controller/BlockController.php
index 2779a9b..7f75d0b 100644
--- a/core/modules/block/src/Controller/BlockController.php
+++ b/core/modules/block/src/Controller/BlockController.php
@@ -11,6 +11,7 @@
 use Drupal\Core\Controller\ControllerBase;
 use Drupal\Core\Extension\ThemeHandlerInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 
 /**
  * Controller routines for admin block routes.
@@ -53,6 +54,11 @@ public static function create(ContainerInterface $container) {
    *   A #type 'page' render array containing the block region demo.
    */
   public function demo($theme) {
+    // Deny access if the theme is not installed or not found.
+    if (!$this->themeHandler->hasUi($theme)) {
+      throw new NotFoundHttpException();
+    }
+
     $page = [
       '#title' => Html::escape($this->themeHandler->getName($theme)),
       '#type' => 'page',
diff --git a/core/modules/block/src/Controller/BlockListController.php b/core/modules/block/src/Controller/BlockListController.php
index 72aa445..b0c5e3f 100644
--- a/core/modules/block/src/Controller/BlockListController.php
+++ b/core/modules/block/src/Controller/BlockListController.php
@@ -8,7 +8,10 @@
 namespace Drupal\block\Controller;
 
 use Drupal\Core\Entity\Controller\EntityListController;
+use Drupal\Core\Extension\ThemeHandlerInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 
 /**
  * Defines a controller to list blocks.
@@ -16,6 +19,32 @@
 class BlockListController extends EntityListController {
 
   /**
+   * The theme handler.
+   *
+   * @var \Drupal\Core\Extension\ThemeHandlerInterface
+   */
+  protected $themeHandler;
+
+  /**
+   * Constructs the BlockListController.
+   *
+   * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
+   *   The theme handler.
+   */
+  public function __construct(ThemeHandlerInterface $theme_handler) {
+    $this->themeHandler = $theme_handler;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('theme_handler')
+    );
+  }
+
+  /**
    * Shows the block administration page.
    *
    * @param string|null $theme
@@ -28,6 +57,11 @@ class BlockListController extends EntityListController {
    */
   public function listing($theme = NULL, Request $request = NULL) {
     $theme = $theme ?: $this->config('system.theme')->get('default');
+    // Deny access if the theme is not installed or not found.
+    if (!$this->themeHandler->hasUi($theme)) {
+      throw new NotFoundHttpException();
+    }
+
     return $this->entityManager()->getListBuilder('block')->render($theme, $request);
   }
 
diff --git a/core/modules/block/src/Plugin/Derivative/ThemeLocalTask.php b/core/modules/block/src/Plugin/Derivative/ThemeLocalTask.php
index 4e337db..ed9d095 100644
--- a/core/modules/block/src/Plugin/Derivative/ThemeLocalTask.php
+++ b/core/modules/block/src/Plugin/Derivative/ThemeLocalTask.php
@@ -50,7 +50,7 @@ public function getDerivativeDefinitions($base_plugin_definition) {
     $default_theme = $this->themeHandler->getDefault();
 
     foreach ($this->themeHandler->listInfo() as $theme_name => $theme) {
-      if ($theme->status) {
+      if ($this->themeHandler->hasUi($theme_name)) {
         $this->derivatives[$theme_name] = $base_plugin_definition;
         $this->derivatives[$theme_name]['title'] = $theme->info['name'];
         $this->derivatives[$theme_name]['route_parameters'] = array('theme' => $theme_name);
diff --git a/core/modules/block/src/Tests/BlockUiTest.php b/core/modules/block/src/Tests/BlockUiTest.php
index 25b7942..34e1c88 100644
--- a/core/modules/block/src/Tests/BlockUiTest.php
+++ b/core/modules/block/src/Tests/BlockUiTest.php
@@ -90,6 +90,10 @@ public function testBlockDemoUiPage() {
     \Drupal::service('theme_handler')->install(array('test_theme'));
     $this->drupalGet('admin/structure/block/demo/test_theme');
     $this->assertEscaped('<strong>Test theme</strong>');
+
+    \Drupal::service('theme_handler')->install(['stable']);
+    $this->drupalGet('admin/structure/block/demo/stable');
+    $this->assertResponse(404, 'Hidden themes that are not the default theme are not supported by the block demo screen');
   }
 
   /**
@@ -136,6 +140,21 @@ function testBlockAdminUiPage() {
     $this->drupalGet('admin/structure/block');
     $element = $this->xpath('//tr[contains(@class, :class)]', [':class' => 'region-title-header']);
     $this->assertTrue(!empty($element));
+
+    // Ensure hidden themes do not appear in the UI. Enable another non base
+    // theme and place the local tasks block.
+    $this->assertTrue(\Drupal::service('theme_handler')->themeExists('classy'), 'The classy base theme is enabled');
+    $this->drupalPlaceBlock('local_tasks_block', ['region' => 'header']);
+    \Drupal::service('theme_installer')->install(['stable']);
+    \Drupal::service('theme_installer')->install(['test_theme']);
+    $this->drupalGet('admin/structure/block');
+    $theme_handler = \Drupal::service('theme_handler');
+    $this->assertLink($theme_handler->getName('classy'));
+    $this->assertLink($theme_handler->getName('test_theme'));
+    $this->assertNoLink($theme_handler->getName('stable'));
+
+    $this->drupalGet('admin/structure/block/list/stable');
+    $this->assertResponse(404, 'Placing blocks through UI is not possible for a hidden base theme.');
   }
 
   /**
diff --git a/core/modules/system/src/Form/ThemeSettingsForm.php b/core/modules/system/src/Form/ThemeSettingsForm.php
index 395c743..a782e15 100644
--- a/core/modules/system/src/Form/ThemeSettingsForm.php
+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -108,13 +108,12 @@ public function buildForm(array $form, FormStateInterface $form_state, $theme =
 
     $themes = $this->themeHandler->listInfo();
 
-    // Deny access if the theme is not installed or not found.
-    if (!empty($theme) && (empty($themes[$theme]) || !$themes[$theme]->status)) {
-      throw new NotFoundHttpException();
-    }
-
     // Default settings are defined in theme_get_setting() in includes/theme.inc
     if ($theme) {
+      // Deny access if the theme is not installed or not found.
+      if (!$this->themeHandler->hasUi($theme)) {
+        throw new NotFoundHttpException();
+      }
       $var = 'theme_' . $theme . '_settings';
       $config_key = $theme . '.settings';
       $themes = $this->themeHandler->listInfo();
diff --git a/core/modules/system/src/Plugin/Derivative/ThemeLocalTask.php b/core/modules/system/src/Plugin/Derivative/ThemeLocalTask.php
index 9c9de19..7193ea5 100644
--- a/core/modules/system/src/Plugin/Derivative/ThemeLocalTask.php
+++ b/core/modules/system/src/Plugin/Derivative/ThemeLocalTask.php
@@ -48,7 +48,7 @@ public static function create(ContainerInterface $container, $base_plugin_id) {
    */
   public function getDerivativeDefinitions($base_plugin_definition) {
     foreach ($this->themeHandler->listInfo() as $theme_name => $theme) {
-      if ($theme->status) {
+      if ($this->themeHandler->hasUi($theme_name)) {
         $this->derivatives[$theme_name] = $base_plugin_definition;
         $this->derivatives[$theme_name]['title'] = $theme->info['name'];
         $this->derivatives[$theme_name]['route_parameters'] = array('theme' => $theme_name);
diff --git a/core/modules/system/src/Tests/System/ThemeTest.php b/core/modules/system/src/Tests/System/ThemeTest.php
index 8b2836b..476a1aa 100644
--- a/core/modules/system/src/Tests/System/ThemeTest.php
+++ b/core/modules/system/src/Tests/System/ThemeTest.php
@@ -52,6 +52,9 @@ function testThemeSettings() {
     $this->assertResponse(404, 'The theme settings form URL for a uninstalled theme could not be found.');
     $this->drupalGet('admin/appearance/settings/' . $this->randomMachineName());
     $this->assertResponse(404, 'The theme settings form URL for a non-existent theme could not be found.');
+    $this->assertTrue(\Drupal::service('theme_installer')->install(['stable']));
+    $this->drupalGet('admin/appearance/settings/stable');
+    $this->assertResponse(404, 'The theme settings form URL for a hidden theme are unavailable.');
 
     // Specify a filesystem path to be used for the logo.
     $file = current($this->drupalGetTestFiles('image'));
@@ -190,6 +193,14 @@ function testThemeSettings() {
     // The logo field should only be present on the global theme settings form.
     $this->assertNoFieldByName('logo_path');
     $this->drupalPostForm(NULL, [], t('Save configuration'));
+
+    // Ensure only vlaid themes are listed in the local tasks.
+    $this->drupalPlaceBlock('local_tasks_block', ['region' => 'header']);
+    $this->drupalGet('admin/appearance/settings');
+    $theme_handler = \Drupal::service('theme_handler');
+    $this->assertLink($theme_handler->getName('classy'));
+    $this->assertLink($theme_handler->getName('bartik'));
+    $this->assertNoLink($theme_handler->getName('stable'));
   }
 
   /**
@@ -255,8 +266,14 @@ function testAdministrationTheme() {
    * Test switching the default theme.
    */
   function testSwitchDefaultTheme() {
+    /** @var \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler */
+    $theme_handler = \Drupal::service('theme_handler');
+    // First, Install Stark and set it as the default theme programmatically.
+    $theme_handler->install(array('stark'));
+    $theme_handler->setDefault('stark');
+
     // Install Bartik and set it as the default theme.
-    \Drupal::service('theme_handler')->install(array('bartik'));
+    $theme_handler->install(array('bartik'));
     $this->drupalGet('admin/appearance');
     $this->clickLink(t('Set as default'));
     $this->assertEqual($this->config('system.theme')->get('default'), 'bartik');
@@ -266,10 +283,10 @@ function testSwitchDefaultTheme() {
     $this->assertText('Bartik(' . t('active tab') . ')', 'Default local task on blocks admin page is the default theme.');
     // Switch back to Stark and test again to test that the menu cache is cleared.
     $this->drupalGet('admin/appearance');
-    // Classy is the first 'Set as default' link.
-    $this->clickLink(t('Set as default'), 0);
+    // Stark is the first 'Set as default' link.
+    $this->clickLink(t('Set as default'));
     $this->drupalGet('admin/structure/block');
-    $this->assertText('Classy(' . t('active tab') . ')', 'Default local task on blocks admin page has changed.');
+    $this->assertText('Stark(' . t('active tab') . ')', 'Default local task on blocks admin page has changed.');
   }
 
   /**
@@ -328,8 +345,8 @@ function testUninstallingThemes() {
     // base theme of bartik.
     $this->assertNoRaw('Uninstall Classy theme', 'A link to uninstall the Classy theme does not appear on the theme settings page.');
 
-    // Change the default theme to stark, stark is third in the list.
-    $this->clickLink(t('Set as default'), 2);
+    // Change the default theme to stark, stark is second in the list.
+    $this->clickLink(t('Set as default'), 1);
 
     // Check that bartik can be uninstalled now.
     $this->assertRaw('Uninstall Bartik theme', 'A link to uninstall the Bartik theme does appear on the theme settings page.');
@@ -344,9 +361,9 @@ function testUninstallingThemes() {
     // Seven is the second in the list.
     $this->clickLink(t('Uninstall'));
     $this->assertRaw('The <em class="placeholder">Seven</em> theme has been uninstalled');
-    // Now uninstall classy.
-    $this->clickLink(t('Uninstall'));
-    $this->assertRaw('The <em class="placeholder">Classy</em> theme has been uninstalled');
+
+    // Check that the classy theme still can't be uninstalled as it is hidden.
+    $this->assertNoRaw('Uninstall Classy theme', 'A link to uninstall the Classy theme does not appear on the theme settings page.');
   }
 
   /**
diff --git a/core/profiles/standard/config/install/block.block.classy_page_title.yml b/core/profiles/standard/config/install/block.block.classy_page_title.yml
deleted file mode 100644
index 4236224..0000000
--- a/core/profiles/standard/config/install/block.block.classy_page_title.yml
+++ /dev/null
@@ -1,17 +0,0 @@
-langcode: en
-status: true
-dependencies:
-  theme:
-    - classy
-id: classy_page_title
-theme: classy
-region: content
-weight: -50
-provider: null
-plugin: page_title_block
-settings:
-  id: page_title_block
-  label: 'Page title'
-  provider: core
-  label_display: '0'
-visibility: {  }
diff --git a/core/themes/classy/classy.info.yml b/core/themes/classy/classy.info.yml
index 2b7705b..5ed9b80 100644
--- a/core/themes/classy/classy.info.yml
+++ b/core/themes/classy/classy.info.yml
@@ -5,6 +5,7 @@ package: Core
 version: VERSION
 core: 8.x
 base theme: false
+hidden: true
 
 libraries:
   - classy/base
diff --git a/core/themes/stable/stable.info.yml b/core/themes/stable/stable.info.yml
index 687c434..960a683 100644
--- a/core/themes/stable/stable.info.yml
+++ b/core/themes/stable/stable.info.yml
@@ -5,3 +5,4 @@ package: Core
 version: VERSION
 core: 8.x
 base theme: false
+hidden: true