diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
index 6dd83f7..422356a 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 +77,24 @@ 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.
+      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 042359f..5c97820 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -88,6 +88,7 @@ public function buildByExtension($extension) {
     }
 
     $libraries = $this->parseLibraryInfo($extension, $path);
+    $this->applyLibrariesOverrides($libraries, $extension, $path);
 
     foreach ($libraries as $id => &$library) {
       if (!isset($library['js']) && !isset($library['css']) && !isset($library['drupalSettings'])) {
@@ -314,6 +315,59 @@ protected function parseLibraryInfo($extension, $path) {
   }
 
   /**
+   * Apply libraries overrides specified for the current active theme.
+   *
+   * @param array $libraries
+   *   The library definition.
+   * @param string $extension
+   *   The extension in which this library was defined.
+   * @param string $path
+   *   The root path of the extension.
+   *
+   * @return array
+   *   The modified library definition.
+   */
+  protected function applyLibrariesOverrides(&$libraries, $extension, $path) {
+    $libraries_overrides = $this->themeManager->getActiveTheme()->getLibrariesOverride();
+    $theme_path = $this->themeManager->getActiveTheme()->getPath();
+    foreach ($libraries as $name => $library) {
+      // Process libraries overrides.
+      foreach ($libraries_overrides as $item => $override) {
+        // Active theme defines an override for this library.
+        if ($item === "$extension/$name") {
+          // The whole library.
+          if ($override) {
+            $libraries[$name]['override'] = $override;
+          }
+          else {
+            $libraries[$name]['override'] = FALSE;
+          }
+        }
+        else if (strpos($item, "$extension/$name/") !== FALSE) {
+          // An asset within the library.
+          list(, , $component, $value) = explode('/', $item, 4);
+          if ($component == 'css') {
+            // SMACSS-category should be incorporated into the component name.
+            list($category, $value) = explode('/', $value, 2);
+            $parents = [$component, $category, $value];
+            $new_parents = [$component, $category, '/' . $theme_path . '/' . $override];
+          }
+          else {
+            $parents = [$component, $value];
+            $new_parents = [$component, '/' . $theme_path . '/' . $override];
+          }
+          // Remove previous component to be overridden.
+          NestedArray::unsetValue($libraries[$name], $parents);
+          if ($override) {
+            // Replace with an override if specified.
+            NestedArray::setValue($libraries[$name], $new_parents, []);
+          }
+        }
+      }
+    }
+  }
+
+  /**
    * Wraps drupal_get_path().
    */
   protected function drupalGetPath($type, $name) {
diff --git a/core/lib/Drupal/Core/Theme/ActiveTheme.php b/core/lib/Drupal/Core/Theme/ActiveTheme.php
index aace6f9..d1970e7 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 components 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'];
   }
 
   /**
@@ -177,4 +186,10 @@ public function getBaseThemes() {
     return $this->baseThemes;
   }
 
+  /**
+   * Returns the libraries or library components overridden by the active theme.
+   */
+  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..6697d9b 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -185,6 +185,25 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
       }
     }
 
+    // Prepare libraries overrides from this theme and ancestor themes.
+    $values['libraries_override'] = array();
+
+    // Grab libraries-overrides from base theme.
+    foreach ($base_themes as $base) {
+      if (!empty($base->info['libraries-override'])) {
+        foreach ($base->info['libraries-override'] as $library => $override) {
+          $values['libraries_override'][$library] = $override;
+        }
+      }
+    }
+
+    // Add libraries-overrides declared by this theme.
+    if (!empty($theme->info['libraries-override'])) {
+      foreach ($theme->info['libraries-override'] as $library => $override) {
+        $values['libraries_override'][$library] = $override;
+      }
+    }
+
     // Do basically the same as the above for libraries
     $values['libraries'] = array();
 
diff --git a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
index 2ddccea..989f56e 100644
--- a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
+++ b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\system\Tests\Asset;
 
+use Drupal\Component\Utility\SafeMarkup;
 use Drupal\simpletest\KernelTestBase;
 
 /**
@@ -42,4 +43,62 @@ public function testElementInfoByTheme() {
     $this->assertTrue($library_discovery->getLibraryByName('test_theme', 'kitten'));
   }
 
+
+  /**
+   * Tests that libraries-overrides 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 component was correctly overridden.
+    $library = $library_discovery->getLibraryByName('classy', 'base');
+    $this->assertAssetInLibraryComponent($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 component was correctly removed.
+    $library = $library_discovery->getLibraryByName('core', 'drupal.dialog');
+    $this->assertNoAssetInLibraryComponent($library['css'], 'drupal.dialog', 'core/misc/dialog.theme.css');
+
+  }
+
+  protected function assertAssetInLibraryComponent($component, $library_name, $asset, $message = NULL) {
+    if (!isset($message)) {
+      $message = SafeMarkup::format('Asset @asset found in library @library', ['@asset' => $asset, '@library' => $library_name]);
+    }
+    foreach ($component as $definition) {
+      if ($asset == $definition['data']) {
+        $this->pass($message);
+        return;
+      }
+    }
+    $this->fail($message);
+  }
+
+  protected function assertNoAssetInLibraryComponent($component, $library_name, $asset, $message = NULL) {
+    if (!isset($message)) {
+      $message = SafeMarkup::format('Asset @asset not found in library @library', ['@asset' => $asset, '@library' => $library_name]);
+    }
+    foreach ($component as $definition) {
+      if ($asset == $definition['data']) {
+        $this->fail($message);
+        return;
+      }
+    }
+    $this->pass($message);
+  }
+
 }
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..d776107 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
@@ -20,3 +20,13 @@ regions:
   content: Content
   left: Left
   right: Right
+
+libraries-override:
+  # 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
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/themes/bartik/bartik.info.yml b/core/themes/bartik/bartik.info.yml
index 3c02894..1af3c21 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'
-