diff --git a/core/lib/Drupal/Core/Asset/AssetResolver.php b/core/lib/Drupal/Core/Asset/AssetResolver.php
index bc8f69f..89408c9 100644
--- a/core/lib/Drupal/Core/Asset/AssetResolver.php
+++ b/core/lib/Drupal/Core/Asset/AssetResolver.php
@@ -138,6 +138,7 @@ public function getCssAssets(AttachedAssetsInterface $assets, $optimize) {
     uasort($css, 'static::sort');
 
     // Allow themes to remove CSS files by CSS files full path and file name.
+    // @todo Remove in Drupal 9.0.x
     if ($stylesheet_remove = $theme_info->getStyleSheetsRemove()) {
       foreach ($css as $key => $options) {
         if (isset($stylesheet_remove[$key])) {
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
index 6dd83f7..bcea7f4 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
@@ -9,8 +9,6 @@
 
 use Drupal\Core\Cache\CacheCollectorInterface;
 use Drupal\Core\Cache\CacheTagsInvalidatorInterface;
-use Drupal\Core\Extension\ModuleHandlerInterface;
-use Drupal\Core\Theme\ThemeManagerInterface;
 
 /**
  * Discovers available asset libraries in Drupal.
@@ -79,7 +77,25 @@ public function getLibrariesByExtension($extension) {
    */
   public function getLibraryByName($extension, $name) {
     $extension = $this->getLibrariesByExtension($extension);
-    return isset($extension[$name]) ? $extension[$name] : FALSE;
+    if (isset($extension[$name])) {
+      // Handle libraries that are marked for override or removal.
+      // @see \Drupal\Core\Asset\LibraryDiscoveryParser::applyLibrariesOverride()
+      if (isset($extension[$name]['override'])) {
+        if ($extension[$name]['override']) {
+          list($new_extension, $new_name) = explode('/', $extension[$name]['override']);
+          $extension[$name] = $this->getLibraryByName($new_extension, $new_name);
+        }
+        else {
+          unset($extension[$name]);
+          return FALSE;
+        }
+      }
+      return $extension[$name];
+    }
+    else {
+      return FALSE;
+    }
+
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 1c08bc9..a8048dd 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\Asset;
 
+use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Asset\Exception\IncompleteLibraryDefinitionException;
 use Drupal\Core\Asset\Exception\InvalidLibraryFileException;
 use Drupal\Core\Asset\Exception\LibraryDefinitionMissingLicenseException;
@@ -88,6 +89,7 @@ public function buildByExtension($extension) {
     }
 
     $libraries = $this->parseLibraryInfo($extension, $path);
+    $libraries = $this->applyLibrariesOverride($libraries, $extension);
 
     foreach ($libraries as $id => &$library) {
       if (!isset($library['js']) && !isset($library['css']) && !isset($library['drupalSettings'])) {
@@ -314,6 +316,71 @@ protected function parseLibraryInfo($extension, $path) {
   }
 
   /**
+   * Apply libraries overrides specified for the current active theme.
+   *
+   * @param array $libraries
+   *   The libraries definitions.
+   * @param string $extension
+   *   The extension in which these libraries are defined.
+   *
+   * @return array
+   *   The modified libraries definitions.
+   */
+  protected function applyLibrariesOverride($libraries, $extension) {
+    $active_theme = $this->themeManager->getActiveTheme();
+    $libraries_overrides = $active_theme->getLibrariesOverride();
+    $theme_path = $active_theme->getPath();
+    foreach ($libraries as $name => $library) {
+      // Process libraries overrides.
+      foreach ($libraries_overrides as $asset => $override) {
+        // Active theme defines an override for this library.
+        if ($asset === "$extension/$name") {
+          // Active theme defines an override for the whole library. Use the
+          // override key to specify that this library will be overridden when
+          // it is called.
+          // @see \Drupal\Core\Asset\LibraryDiscovery::getLibraryByName()
+          if ($override) {
+            $libraries[$name]['override'] = $override;
+          }
+          else {
+            $libraries[$name]['override'] = FALSE;
+          }
+        }
+        else if (strpos($asset, "$extension/$name/") !== FALSE) {
+          // Active theme defines an override for an asset within this library.
+          // Throw an exception if the asset is not properly specified.
+          if (substr_count($asset, '/') < 3) {
+            throw new \LogicException(SafeMarkup::format('Library asset %asset is not correctly specified. It should be in the form "extension/library_name/sub_key/path/to/asset.js".', ['%asset' => $asset]));
+          }
+          list(, , $sub_key, $value) = explode('/', $asset, 4);
+          if ($sub_key === 'drupalSettings') {
+            // drupalSettings may not be overridden.
+            throw new \LogicException(SafeMarkup::format('drupalSettings may not be overridden in libraries-override. Trying to override %asset. Use hook_library_info_alter() instead.', ['%asset' =>$asset]));
+          }
+          else if ($sub_key === 'css') {
+            // SMACSS category should be incorporated into the asset name.
+            list($category, $value) = explode('/', $value, 2);
+            $parents = [$sub_key, $category, $value];
+            $new_parents = [$sub_key, $category, '/' . $theme_path . '/' . $override];
+          }
+          else {
+            $parents = [$sub_key, $value];
+            $new_parents = [$sub_key, '/' . $theme_path . '/' . $override];
+          }
+          // Remove asset to be overridden, but keep its attributes.
+          $attributes = NestedArray::getValue($libraries[$name], $parents);
+          NestedArray::unsetValue($libraries[$name], $parents);
+          if ($override) {
+            // Replace with an override if specified.
+            NestedArray::setValue($libraries[$name], $new_parents, $attributes);
+          }
+        }
+      }
+    }
+    return $libraries;
+  }
+
+  /**
    * Wraps drupal_get_path().
    */
   protected function drupalGetPath($type, $name) {
diff --git a/core/lib/Drupal/Core/Theme/ActiveTheme.php b/core/lib/Drupal/Core/Theme/ActiveTheme.php
index aace6f9..1414de0 100644
--- a/core/lib/Drupal/Core/Theme/ActiveTheme.php
+++ b/core/lib/Drupal/Core/Theme/ActiveTheme.php
@@ -74,6 +74,13 @@ class ActiveTheme {
   protected $libraries;
 
   /**
+   * The libraries or library assets overridden by the theme.
+   *
+   * @var array
+   */
+  protected $librariesOverride;
+
+  /**
    * Constructs an ActiveTheme object.
    *
    * @param array $values
@@ -88,6 +95,7 @@ public function __construct(array $values) {
       'libraries' => [],
       'extension' => 'html.twig',
       'base_themes' => [],
+      'libraries_override' => [],
     ];
 
     $this->name = $values['name'];
@@ -98,6 +106,7 @@ public function __construct(array $values) {
     $this->libraries = $values['libraries'];
     $this->extension = $values['extension'];
     $this->baseThemes = $values['base_themes'];
+    $this->librariesOverride = $values['libraries_override'];
   }
 
   /**
@@ -160,6 +169,8 @@ public function getLibraries() {
    * Returns the removed stylesheets by the theme.
    *
    * @return mixed
+   *
+   * @deprecated in Drupal 8.0.0, will be removed before Drupal 9.0.0.
    */
   public function getStyleSheetsRemove() {
     return $this->styleSheetsRemove;
@@ -177,4 +188,24 @@ public function getBaseThemes() {
     return $this->baseThemes;
   }
 
+  /**
+   * Returns the libraries or library assets overridden by the active theme.
+   *
+   * The array returned is in the form:
+   * @code
+   * [
+   *   'core/drupal.collapse' => 'test_theme/collapse',
+   *   'core/drupal.progress' => FALSE,
+   *   'classy/base/css/theme/css/layout.css' => 'css/test_theme_layout.css',
+   *   'core/drupal.dialog/css/theme/misc/dialog.theme.css' => FALSE,
+   *   'core/jquery/js/assets/vendor/jquery/jquery.min.js' => 'js/collapse.js',
+   * ]
+   * @endcode
+   *
+   * @return array
+   */
+  public function getLibrariesOverride() {
+    return $this->librariesOverride;
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
index a77271e..d953d49 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -161,27 +161,27 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
     $values['path'] = $theme_path;
     $values['name'] = $theme->getName();
 
-    // Prepare stylesheets from this theme as well as all ancestor themes.
-    // We work it this way so that we can have child themes remove CSS files
-    // easily from parent.
-    $values['stylesheets_remove'] = array();
+    // @todo Remove in Drupal 9.0.x
+    $values['stylesheets_remove'] = $this->prepareStylesheetsRemove($theme, $base_themes);
 
-    // Grab stylesheets from base theme.
+    // Prepare libraries overrides from this theme and ancestor themes.
+    // This allows child themes to easily remove CSS files from base themes and
+    // modules.
+    $values['libraries_override'] = array();
+
+    // Get libraries-override declared by base theme.
     foreach ($base_themes as $base) {
-      $base_theme_path = $base->getPath();
-      if (!empty($base->info['stylesheets-remove'])) {
-        foreach ($base->info['stylesheets-remove'] as $css_file) {
-          $css_file = $this->resolveStyleSheetPlaceholders($css_file);
-          $values['stylesheets_remove'][$css_file] = $css_file;
+      if (!empty($base->info['libraries-override'])) {
+        foreach ($base->info['libraries-override'] as $library => $override) {
+          $values['libraries_override'][$library] = $override;
         }
       }
     }
 
-    // Add stylesheets used by this theme.
-    if (!empty($theme->info['stylesheets-remove'])) {
-      foreach ($theme->info['stylesheets-remove'] as $css_file) {
-        $css_file = $this->resolveStyleSheetPlaceholders($css_file);
-        $values['stylesheets_remove'][$css_file] = $css_file;
+    // Add libraries-override declared by this theme.
+    if (!empty($theme->info['libraries-override'])) {
+      foreach ($theme->info['libraries-override'] as $library => $override) {
+        $values['libraries_override'][$library] = $override;
       }
     }
 
@@ -238,6 +238,8 @@ protected function getExtensions() {
    *
    * @return string
    *   CSS file where placeholders are replaced.
+   *
+   * @todo Remove in Drupal 9.0.x
    */
   protected function resolveStyleSheetPlaceholders($css_file) {
     $token_candidate = explode('/', $css_file)[0];
@@ -253,4 +255,43 @@ protected function resolveStyleSheetPlaceholders($css_file) {
       return str_replace($token_candidate, $extensions[$token]->getPath(), $css_file);
     }
   }
+
+  /**
+   * Prepares stylesheets-remove specified in the *.info.yml file.
+   *
+   * @param \Drupal\Core\Extension\Extension $theme
+   *   The theme extension object.
+   * @param \Drupal\Core\Extension\Extension[] $base_themes
+   *   An array of base themes.
+   *
+   * @return array
+   *
+   * @todo Remove in Drupal 9.0.x
+   */
+  protected function prepareStylesheetsRemove(Extension $theme, $base_themes) {
+    // Prepare stylesheets from this theme as well as all ancestor themes.
+    // We work it this way so that we can have child themes remove CSS files
+    // easily from parent.
+    $stylesheets_remove = array();
+    // Grab stylesheets from base theme.
+     foreach ($base_themes as $base) {
+       $base_theme_path = $base->getPath();
+       if (!empty($base->info['stylesheets-remove'])) {
+         foreach ($base->info['stylesheets-remove'] as $css_file) {
+           $css_file = $this->resolveStyleSheetPlaceholders($css_file);
+           $stylesheets_remove[$css_file] = $css_file;
+         }
+       }
+     }
+
+    // Add stylesheets used by this theme.
+    if (!empty($theme->info['stylesheets-remove'])) {
+      foreach ($theme->info['stylesheets-remove'] as $css_file) {
+        $css_file = $this->resolveStyleSheetPlaceholders($css_file);
+        $stylesheets_remove[$css_file] = $css_file;
+      }
+    }
+    return $stylesheets_remove;
+  }
+
 }
diff --git a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
index 2ddccea..2f1e6fb 100644
--- a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
+++ b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
@@ -7,6 +7,9 @@
 
 namespace Drupal\system\Tests\Asset;
 
+use Drupal\Component\Utility\SafeMarkup;
+use Drupal\Core\Extension\Extension;
+use Drupal\Core\Theme\ActiveTheme;
 use Drupal\simpletest\KernelTestBase;
 
 /**
@@ -42,4 +45,125 @@ public function testElementInfoByTheme() {
     $this->assertTrue($library_discovery->getLibraryByName('test_theme', 'kitten'));
   }
 
+  /**
+   * Tests that libraries-override are applied to library definitions.
+   */
+  public function testLibrariesOverride() {
+    /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
+    $theme_initializer = $this->container->get('theme.initialization');
+
+    /** @var \Drupal\Core\Theme\ThemeManagerInterface $theme_manager */
+    $theme_manager = $this->container->get('theme.manager');
+
+    /** @var \Drupal\Core\Render\ElementInfoManagerInterface $element_info */
+    $library_discovery = $this->container->get('library.discovery');
+
+    $theme_manager->setActiveTheme($theme_initializer->getActiveThemeByName('test_theme'));
+
+    // Assert that entire library was correctly overridden.
+    $this->assertEqual($library_discovery->getLibraryByName('core', 'drupal.collapse'), $library_discovery->getLibraryByName('test_theme', 'collapse'), 'Entire library correctly overridden.');
+
+    // Assert that library asset was correctly overridden.
+    $library = $library_discovery->getLibraryByName('classy', 'base');
+    $this->assertAssetInLibrary($library['css'], 'base', 'core/modules/system/tests/themes/test_theme/css/test_theme_layout.css');
+
+    // Assert that entire library was correctly removed.
+    $this->assertFalse($library_discovery->getLibraryByName('core', 'drupal.progress'), 'Entire library correctly removed.');
+
+    // Assert that library asset was correctly removed.
+    $library = $library_discovery->getLibraryByName('core', 'drupal.dialog');
+    $this->assertNoAssetInLibrary($library['css'], 'drupal.dialog', 'core/misc/dialog.theme.css');
+
+    // Assert that overridden library asset still retains attributes.
+    $library = $library_discovery->getLibraryByName('core', 'jquery');
+    foreach ($library['js'] as $definition) {
+      if ($definition['data'] == 'core/modules/system/tests/themes/test_theme/js/collapse.js') {
+        $this->assertTrue($definition['minified'] && $definition['weight'] == -20, 'Previous attributes retained');
+        break;
+      }
+    }
+  }
+
+  /**
+   * Tests libraries-override on drupalSettings.
+   */
+  public function testLibrariesOverrideDrupalSettings() {
+    $this->container->get('theme_handler')->install(['test_theme_libraries_override_with_drupal_settings']);
+
+    /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
+    $theme_initializer = $this->container->get('theme.initialization');
+
+    /** @var \Drupal\Core\Theme\ThemeManagerInterface $theme_manager */
+    $theme_manager = $this->container->get('theme.manager');
+
+    /** @var \Drupal\Core\Render\ElementInfoManagerInterface $element_info */
+    $library_discovery = $this->container->get('library.discovery');
+
+    $theme_manager->setActiveTheme($theme_initializer->getActiveThemeByName('test_theme_libraries_override_with_drupal_settings'));
+
+    // Assert that drupalSettings cannot be overridden and throws an exception.
+    try {
+      $library_discovery->getLibraryByName('core', 'drupal.ajax');
+      $this->fail('Throw LogicException when trying to override drupalSettings');
+    }
+    catch (\LogicException $e) {
+      $expected_message = 'drupalSettings may not be overridden in libraries-override. Trying to override <em class="placeholder">core/drupal.ajax/drupalSettings/ajaxPageState</em>. Use hook_library_info_alter() instead.';
+      $this->assertEqual($e->getMessage(), $expected_message, 'Throw LogicException when trying to override drupalSettings');
+    }
+  }
+
+  /**
+   * Tests libraries-override on malformed assets.
+   */
+  public function testLibrariesOverrideMalformedAsset() {
+    $this->container->get('theme_handler')->install(['test_theme_libraries_override_with_invalid_asset']);
+
+    /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
+    $theme_initializer = $this->container->get('theme.initialization');
+
+    /** @var \Drupal\Core\Theme\ThemeManagerInterface $theme_manager */
+    $theme_manager = $this->container->get('theme.manager');
+
+    /** @var \Drupal\Core\Render\ElementInfoManagerInterface $element_info */
+    $library_discovery = $this->container->get('library.discovery');
+
+    $theme_manager->setActiveTheme($theme_initializer->getActiveThemeByName('test_theme_libraries_override_with_invalid_asset'));
+
+    // Assert that improperly formed asset "specs" throw an exception.
+    try {
+      $library_discovery->getLibraryByName('core', 'drupal.dialog');
+      $this->fail('Throw LogicException when specifying invalid override');
+    }
+    catch (\LogicException $e) {
+      $expected_message = 'Library asset <em class="placeholder">core/drupal.dialog/css</em> is not correctly specified. It should be in the form "extension/library_name/sub_key/path/to/asset.js".';
+      $this->assertEqual($e->getMessage(), $expected_message, 'Throw LogicException when specifying invalid override');
+    }
+  }
+
+  protected function assertAssetInLibrary($library_item, $library_name, $asset, $message = NULL) {
+    if (!isset($message)) {
+      $message = SafeMarkup::format('Asset @asset found in library @library', ['@asset' => $asset, '@library' => $library_name]);
+    }
+    foreach ($library_item as $definition) {
+      if ($asset == $definition['data']) {
+        $this->pass($message);
+        return;
+      }
+    }
+    $this->fail($message);
+  }
+
+  protected function assertNoAssetInLibrary($library_item, $library_name, $asset, $message = NULL) {
+    if (!isset($message)) {
+      $message = SafeMarkup::format('Asset @asset not found in library @library', ['@asset' => $asset, '@library' => $library_name]);
+    }
+    foreach ($library_item as $definition) {
+      if ($asset == $definition['data']) {
+        $this->fail($message);
+        return;
+      }
+    }
+    $this->pass($message);
+  }
+
 }
diff --git a/core/modules/system/tests/themes/test_theme/css/collapse.css b/core/modules/system/tests/themes/test_theme/css/collapse.css
new file mode 100644
index 0000000..c781141
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/css/collapse.css
@@ -0,0 +1,4 @@
+/**
+ * @file
+ * Test CSS asset file for test_theme.theme
+ */
diff --git a/core/modules/system/tests/themes/test_theme/js/collapse.js b/core/modules/system/tests/themes/test_theme/js/collapse.js
new file mode 100644
index 0000000..bfb509c
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/js/collapse.js
@@ -0,0 +1,4 @@
+/**
+ * @file
+ * Test JS asset file for test_theme.theme
+ */
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.info.yml b/core/modules/system/tests/themes/test_theme/test_theme.info.yml
index 7ddcb2d..e285cd7 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.info.yml
+++ b/core/modules/system/tests/themes/test_theme/test_theme.info.yml
@@ -14,8 +14,18 @@ description: 'Theme for testing the theme system'
 version: VERSION
 base theme: classy
 core: 8.x
-stylesheets-remove:
-  - '@system/css/system.module.css'
+libraries-override:
+  system/base/css/component/css/system.module.css: false
+  # Replace an entire library.
+  core/drupal.collapse: test_theme/collapse
+  # Remove an entire library.
+  core/drupal.progress: false
+  # Replace one particular library asset with another.
+  classy/base/css/theme/css/layout.css: css/test_theme_layout.css
+  # Remove one particular asset.
+  core/drupal.dialog/css/theme/misc/dialog.theme.css: false
+  # It works for JS as well.
+  core/jquery/js/assets/vendor/jquery/jquery.min.js: js/collapse.js
 regions:
   content: Content
   left: Left
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml b/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml
new file mode 100644
index 0000000..69d07fb
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml
@@ -0,0 +1,7 @@
+collapse:
+  js:
+    js/collapse.js: { }
+
+  css:
+    base:
+      css/collapse.css: { }
diff --git a/core/modules/system/tests/themes/test_theme_libraries_override_with_drupal_settings/test_theme_libraries_override_with_drupal_settings.info.yml b/core/modules/system/tests/themes/test_theme_libraries_override_with_drupal_settings/test_theme_libraries_override_with_drupal_settings.info.yml
new file mode 100644
index 0000000..0939c27
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_libraries_override_with_drupal_settings/test_theme_libraries_override_with_drupal_settings.info.yml
@@ -0,0 +1,9 @@
+name: 'Test theme libraries-override'
+type: theme
+description: 'Theme with drupalSettings libraries-override'
+version: VERSION
+base theme: classy
+core: 8.x
+libraries-override:
+  # drupalSettings libraries override. Should throw a \LogicException.
+  core/drupal.ajax/drupalSettings/ajaxPageState: { }
diff --git a/core/modules/system/tests/themes/test_theme_libraries_override_with_invalid_asset/test_theme_libraries_override_with_invalid_asset.info.yml b/core/modules/system/tests/themes/test_theme_libraries_override_with_invalid_asset/test_theme_libraries_override_with_invalid_asset.info.yml
new file mode 100644
index 0000000..ecba74e
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_libraries_override_with_invalid_asset/test_theme_libraries_override_with_invalid_asset.info.yml
@@ -0,0 +1,9 @@
+name: 'Test theme libraries-override'
+type: theme
+description: 'Theme with invalid libraries-override asset spec.'
+version: VERSION
+base theme: classy
+core: 8.x
+libraries-override:
+  # A malformed library asset name. Should throw a \LogicException.
+  core/drupal.dialog/css: false
diff --git a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
index 9865cb2..2f5ed21 100644
--- a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
@@ -73,6 +73,15 @@ protected function setUp() {
 
     $this->moduleHandler = $this->getMock('Drupal\Core\Extension\ModuleHandlerInterface');
     $this->themeManager = $this->getMock('Drupal\Core\Theme\ThemeManagerInterface');
+    $mock_active_theme = $this->getMockBuilder('Drupal\Core\Theme\ActiveTheme')
+      ->disableOriginalConstructor()
+      ->getMock();
+    $mock_active_theme->expects($this->any())
+      ->method('getLibrariesOverride')
+      ->willReturn([]);
+    $this->themeManager->expects($this->any())
+      ->method('getActiveTheme')
+      ->willReturn($mock_active_theme);
     $this->libraryDiscoveryParser = new TestLibraryDiscoveryParser($this->root, $this->moduleHandler, $this->themeManager);
   }
 
diff --git a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
index 4a9101d..71a71f4 100644
--- a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
+++ b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
@@ -93,7 +93,7 @@ public function testGetRegistryForModule() {
       'engine' => 'twig',
       'owner' => 'twig',
       'stylesheets_remove' => [],
-      'stylesheets_override' => [],
+      'libraries_override' => [],
       'libraries' => [],
       'extension' => '.twig',
       'base_themes' => [],
diff --git a/core/themes/bartik/bartik.info.yml b/core/themes/bartik/bartik.info.yml
index fb8f2fb..fa73bcb 100644
--- a/core/themes/bartik/bartik.info.yml
+++ b/core/themes/bartik/bartik.info.yml
@@ -5,10 +5,10 @@ description: 'A flexible, recolorable theme with many regions and a responsive,
 package: Core
 version: VERSION
 core: 8.x
-stylesheets-remove:
-  - '@classy/css/layout.css'
 libraries:
   - bartik/global-styling
+libraries-override:
+  classy/base/css/theme/css/layout.css: false
 ckeditor_stylesheets:
   - css/base/elements.css
   - css/components/captions.css
@@ -35,4 +35,3 @@ regions:
   footer_third: 'Footer third'
   footer_fourth: 'Footer fourth'
   footer_fifth: 'Footer fifth'
-
diff --git a/core/themes/seven/seven.info.yml b/core/themes/seven/seven.info.yml
index 99a3ec1..daf74a0 100644
--- a/core/themes/seven/seven.info.yml
+++ b/core/themes/seven/seven.info.yml
@@ -8,9 +8,9 @@ version: VERSION
 core: 8.x
 libraries:
  - seven/global-styling
-stylesheets-remove:
-  - core/assets/vendor/jquery.ui/themes/base/dialog.css
-  - '@classy/css/layout.css'
+libraries-override:
+  core/jquery.ui.dialog/css/component/assets/vendor/jquery.ui/themes/base/dialog.css: false
+  classy/base/css/theme/css/layout.css: false
 quickedit_stylesheets:
   - css/components/quickedit.css
 regions:
diff --git a/core/themes/stark/stark.info.yml b/core/themes/stark/stark.info.yml
index 69337d5..ed70cd6 100644
--- a/core/themes/stark/stark.info.yml
+++ b/core/themes/stark/stark.info.yml
@@ -6,5 +6,5 @@ version: VERSION
 core: 8.x
 libraries:
   - stark/global-styling
-stylesheets-remove:
-  - core/assets/vendor/normalize-css/normalize.css
+libraries-override:
+  core/normalize/css/base/assets/vendor/normalize-css/normalize.css: false