diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index e7da564..7791869 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -11,6 +11,7 @@
 use Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException;
 use Drupal\Core\Asset\Exception\InvalidLibraryFileException;
 use Drupal\Core\Asset\Exception\LibraryDefinitionMissingLicenseException;
+use Drupal\Core\Extension\Extension;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Theme\ThemeManagerInterface;
 use Drupal\Component\Serialization\Exception\InvalidDataTypeException;
@@ -335,55 +336,48 @@ protected function parseLibraryInfo($extension, $path) {
    */
   protected function applyLibrariesOverride($libraries, $extension) {
     $active_theme = $this->themeManager->getActiveTheme();
-    $libraries_overrides = $active_theme->getLibrariesOverride();
-    foreach ($libraries as $library_name => $library) {
-      // Process libraries overrides.
-      $namespaced_library_name = "$extension/$library_name";
-      foreach ($libraries_overrides as $definition => $override) {
-        // Active theme defines an override for this library.
-        if ($this->isEntireLibrary($definition, $namespaced_library_name)) {
-          // The active theme defines an override for the whole library. Use the
-          // override key to specify that this library will be overridden when
-          // it is called.
-          // @see \Drupal\Core\Asset\LibraryDiscovery::getLibraryByName()
-          if ($override) {
-            $libraries[$library_name]['override'] = $override;
-          }
-          else {
-            $libraries[$library_name]['override'] = FALSE;
-          }
-        }
-        elseif ($this->isPartOfLibrary($definition, $namespaced_library_name)) {
-          // Active theme defines an override for an asset within this library.
-          // Throw an exception if the asset is not properly specified.
-          if (substr_count($definition, '/') < 3) {
-            throw new InvalidLibrariesOverrideSpecificationException(sprintf('Library asset %s is not correctly specified. It should be in the form "extension/library_name/sub_key/path/to/asset.js".', $definition));
-          }
-          list(, , $sub_key, $value) = explode('/', $definition, 4);
-          if ($sub_key === 'drupalSettings') {
-            // drupalSettings may not be overridden.
-            throw new InvalidLibrariesOverrideSpecificationException(sprintf('drupalSettings may not be overridden in libraries-override. Trying to override %s. Use hook_library_info_alter() instead.', $definition));
-          }
-          elseif ($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, $override];
-          }
-          else {
-            $parents = [$sub_key, $value];
-            $new_parents = [$sub_key, $override];
+    // ActiveTheme::getLibrariesOverride() returns libraries-overrides for the
+    // current theme as well as all its base themes.
+    $all_libraries_overrides = $active_theme->getLibrariesOverride();
+    foreach ($all_libraries_overrides as $theme_path => $libraries_overrides) {
+      foreach ($libraries as $library_name => $library) {
+        // Process libraries overrides.
+        if (isset($libraries_overrides["$extension/$library_name"])) {
+          // Active theme defines an override for this library.
+          $override_definition = $libraries_overrides["$extension/$library_name"];
+          if (is_string($override_definition) || $override_definition === FALSE) {
+            // A string or boolean definition implies an override (or removal)
+            // for the whole library. Use the override key to specify that this
+            // library will be overridden when it is called.
+            // @see \Drupal\Core\Asset\LibraryDiscovery::getLibraryByName()
+            if ($override_definition) {
+              $libraries[$library_name]['override'] = $override_definition;
+            }
+            else {
+              $libraries[$library_name]['override'] = FALSE;
+            }
           }
-          // Get the attributes of the asset to be overridden. If the key does
-          // not exist, then throw an exception.
-          $key_exists = NULL;
-          $attributes = NestedArray::getValue($libraries[$library_name], $parents, $key_exists);
-          if ($key_exists) {
-            // Remove asset to be overridden, but keep its attributes.
-            NestedArray::unsetValue($libraries[$library_name], $parents);
-            if ($override) {
-              // Replace with an override if specified.
-              NestedArray::setValue($libraries[$library_name], $new_parents, $attributes);
+          elseif (is_array($override_definition)) {
+            // An array definition implies an override for an asset within this
+            // library.
+            foreach ($override_definition as $sub_key => $value) {
+              // Throw an exception if the asset is not properly specified.
+              if (!is_array($value)) {
+                throw new InvalidLibrariesOverrideSpecificationException(sprintf('Library asset %s is not correctly specified. It should be in the form "extension/library_name/sub_key/path/to/asset.js".', "$extension/$library_name/$sub_key"));
+              }
+              if ($sub_key === 'drupalSettings') {
+                // drupalSettings may not be overridden.
+                throw new InvalidLibrariesOverrideSpecificationException(sprintf('drupalSettings may not be overridden in libraries-override. Trying to override %s. Use hook_library_info_alter() instead.', "$extension/$library_name/$sub_key"));
+              }
+              elseif ($sub_key === 'css') {
+                // SMACSS category should be incorporated into the asset name.
+                foreach ($value as $category => $overrides) {
+                  $this->setOverrideValue($libraries[$library_name], [$sub_key, $category], $overrides, $theme_path);
+                }
+              }
+              else {
+                $this->setOverrideValue($libraries[$library_name], [$sub_key], $value, $theme_path);
+              }
             }
           }
         }
@@ -415,31 +409,57 @@ protected function isValidUri($string) {
   }
 
   /**
-   * Checks if the specified definition asset is a full library.
+   * Overrides the specified library asset.
    *
-   * @param string $definition
-   *   The asset definition being checked.
-   * @param string $library_name
-   *   The name of the library.
-   * @return bool
-   *   Returns true if $definition is a full library specified by $library_name.
+   * @param array $library
+   *   The containing library definition.
+   * @param array $sub_key
+   *   An array containing the sub-keys specifying the library asset, e.g.
+   *   @code['js']@endcode or @code['css', 'component']@endcode
+   * @param array $overrides
+   *   Specifies the overrides, this is an array where the key is the asset to
+   *   be overridden while the value is overriding asset.
    */
-  protected function isEntireLibrary($definition, $library_name) {
-    return $definition === $library_name;
+  protected function setOverrideValue(array &$library, array $sub_key, array $overrides, $theme_path) {
+    foreach ($overrides as $original => $replacement) {
+      // Get the attributes of the asset to be overridden. If the key does
+      // not exist, then throw an exception.
+      $key_exists = NULL;
+      $parents = array_merge($sub_key, [$original]);
+      // Ensure the replacement path is relative to drupal root.
+      $replacement = $this->resolveThemeAssetPath($theme_path, $replacement);
+      $new_parents = array_merge($sub_key, [$replacement]);
+      $attributes = NestedArray::getValue($library, $parents, $key_exists);
+      if ($key_exists) {
+        // Remove asset to be overridden, but keep its attributes.
+        NestedArray::unsetValue($library, $parents);
+        if ($replacement) {
+          // Replace with an override if specified.
+          NestedArray::setValue($library, $new_parents, $attributes);
+        }
+      }
+    }
   }
 
   /**
-   * Determines if the specified asset definition is a part of a library.
+   * Ensures that a full path is returned for an overriding theme asset.
    *
-   * @param string $definition
-   *   The asset definition being checked.
-   * @param string $library_name
-   *   The name of the library.
-   * @return bool
-   *   Returns true if the definition is part of the library, false otherwise.
+   * @param string $theme_path
+   *   The theme or base theme.
+   * @param string $overriding_asset
+   *   The overriding library asset.
+   *
+   * @return string
+   *   A fully resolved theme asset path relative to the Drupal directory.
    */
-  protected function isPartOfLibrary($definition, $library_name) {
-    return strpos($definition, "$library_name/") === 0;
+  protected function resolveThemeAssetPath($theme_path, $overriding_asset) {
+    if ($overriding_asset[0] !== '/' && !$this->isValidUri($overriding_asset)) {
+      // The destination is not an absolute path and it's not a URI (e.g.
+      // public://generated_js/example.js or http://example.com/js/my_js.js), so
+      // it's relative to the theme.
+      return '/' . $theme_path . '/' . $overriding_asset;
+    }
+    return $overriding_asset;
   }
 
 }
diff --git a/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
index 8a240d5..74ac35c 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -173,7 +173,7 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
     foreach ($base_themes as $base) {
       if (!empty($base->info['libraries-override'])) {
         foreach ($base->info['libraries-override'] as $library => $override) {
-          $values['libraries_override'][$library] = $override ? $this->resolveThemeAssetPath($base, $library, $override) : $override;
+          $values['libraries_override'][$base->getPath()][$library] = $override;
         }
       }
     }
@@ -181,7 +181,7 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
     // Add libraries overrides declared by this theme.
     if (!empty($theme->info['libraries-override'])) {
       foreach ($theme->info['libraries-override'] as $library => $override) {
-        $values['libraries_override'][$library] = $override ? $this->resolveThemeAssetPath($theme, $library, $override) : $override;
+        $values['libraries_override'][$theme->getPath()][$library] = $override;
       }
     }
 
@@ -298,42 +298,4 @@ protected function prepareStylesheetsRemove(Extension $theme, $base_themes) {
     return $stylesheets_remove;
   }
 
-  /**
-   * Ensures that a full path is returned for an overriding theme asset.
-   *
-   * If the specified $source is a library, then the $destination is returned
-   * without change.
-   *
-   * @param \Drupal\Core\Extension\Extension $theme
-   *   The theme or base theme.
-   * @param string $source
-   *   The source asset path, i.e. the library to be replaced.
-   * @param string $destination
-   *   The library or library asset replacing the source.
-   *
-   * @return string
-   *   A fully resolved theme asset path relative to the Drupal directory.
-   */
-  protected function resolveThemeAssetPath(Extension $theme, $source, $destination) {
-    // Full library definitions don't need to be resolved.
-    if (count(explode('/', $source)) === 2) {
-      return $destination;
-    }
-    // The destination is not an absolute path and it's not a URI (e.g.
-    // public://generated_js/example.js or http://example.com/js/my_js.js), so
-    // it's relative to the theme.
-    if ($destination[0] !== '/' && !$this->isValidUri($destination)) {
-      $theme_path = $theme->getPath();
-      return '/' . $theme_path . '/' . $destination;
-    }
-    return $destination;
-  }
-
-  /**
-   * Determines if the supplied string is a valid URI.
-   */
-  protected function isValidUri($string) {
-    return count(explode('://', $string)) === 2;
-  }
-
 }
diff --git a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
index f2560fc..07a2ecb 100644
--- a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
+++ b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
@@ -30,7 +30,7 @@ class LibraryDiscoveryIntegrationTest extends KernelTestBase {
   protected function setUp() {
     parent::setUp();
 
-    $this->container->get('theme_handler')->install(['test_theme', 'classy']);
+    $this->container->get('theme_installer')->install(['test_theme', 'classy']);
     $this->libraryDiscovery = $this->container->get('library.discovery');
   }
 
@@ -46,14 +46,33 @@ public function testElementInfoByTheme() {
    * Tests that libraries-override are applied to library definitions.
    */
   public function testLibrariesOverride() {
+    // Assert some classy libraries exist that we are going to override.
+    $this->activateTheme('classy');
+    $this->assertAssetInLibrary('core/themes/classy/css/components/button.css', 'classy', 'base', 'css');
+    $this->assertAssetInLibrary('core/themes/classy/css/components/collapse-processed.css', 'classy', 'base', 'css');
+    $this->assertAssetInLibrary('core/themes/classy/css/components/container-inline.css', 'classy', 'base', 'css');
+    $this->assertAssetInLibrary('core/themes/classy/css/components/details.css', 'classy', 'base', 'css');
+
+    // Confirmatory asserts on other libraries to be removed or overridden.
+    $this->assertTrue($this->libraryDiscovery->getLibraryByName('core', 'drupal.progress'), 'Confirmatory test on "core/drupal.progress"');
+    $this->assertAssetInLibrary('core/misc/dialog.theme.css', 'core', 'drupal.dialog', 'css');
+
     // Activate test theme that defines libraries overrides.
     $this->activateTheme('test_theme');
 
     // Assert that entire library was correctly overridden.
     $this->assertEqual($this->libraryDiscovery->getLibraryByName('core', 'drupal.collapse'), $this->libraryDiscovery->getLibraryByName('test_theme', 'collapse'), 'Entire library correctly overridden.');
 
-    // Assert that library asset was correctly overridden.
-    $this->assertAssetInLibrary('core/modules/system/tests/themes/test_theme/css/test_theme_layout.css', 'classy', 'base', 'css');
+    // Assert that classy library assets were correctly overridden.
+    $this->assertNoAssetInLibrary('core/themes/classy/css/components/button.css', 'classy', 'base', 'css');
+    $this->assertNoAssetInLibrary('core/themes/classy/css/components/collapse-processed.css', 'classy', 'base', 'css');
+    $this->assertNoAssetInLibrary('core/themes/classy/css/components/container-inline.css', 'classy', 'base', 'css');
+    $this->assertNoAssetInLibrary('core/themes/classy/css/components/details.css', 'classy', 'base', 'css');
+
+    $this->assertAssetInLibrary('core/modules/system/tests/themes/test_theme/css/my-button.css', 'classy', 'base', 'css');
+    $this->assertAssetInLibrary('core/modules/system/tests/themes/test_theme/css/my-collapse-processed.css', 'classy', 'base', 'css');
+    $this->assertAssetInLibrary('themes/my_theme/css/my-container-inline.css', 'classy', 'base', 'css');
+    $this->assertAssetInLibrary('themes/my_theme/css/my-details.css', 'classy', 'base', 'css');
 
     // Assert that entire library was correctly removed.
     $this->assertFalse($this->libraryDiscovery->getLibraryByName('core', 'drupal.progress'), 'Entire library correctly removed.');
@@ -84,7 +103,7 @@ public function testLibrariesOverrideDrupalSettings() {
       $this->fail('Throw Exception when trying to override drupalSettings');
     }
     catch (InvalidLibrariesOverrideSpecificationException $e) {
-      $expected_message = 'drupalSettings may not be overridden in libraries-override. Trying to override core/drupal.ajax/drupalSettings/ajaxPageState. Use hook_library_info_alter() instead.';
+      $expected_message = 'drupalSettings may not be overridden in libraries-override. Trying to override core/drupal.ajax/drupalSettings. Use hook_library_info_alter() instead.';
       $this->assertEqual($e->getMessage(), $expected_message, 'Throw Exception when trying to override drupalSettings');
     }
   }
@@ -150,7 +169,7 @@ public function testBaseThemeLibrariesOverrideInSubTheme() {
    *   The name of the theme to be activated.
    */
   protected function activateTheme($theme_name) {
-    $this->container->get('theme_handler')->install([$theme_name]);
+    $this->container->get('theme_installer')->install([$theme_name]);
 
     /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
     $theme_initializer = $this->container->get('theme.initialization');
@@ -159,6 +178,8 @@ protected function activateTheme($theme_name) {
     $theme_manager = $this->container->get('theme.manager');
 
     $theme_manager->setActiveTheme($theme_initializer->getActiveThemeByName($theme_name));
+
+    $this->libraryDiscovery->clearCachedDefinitions();
   }
 
   /**
@@ -180,7 +201,7 @@ protected function activateTheme($theme_name) {
    */
   protected function assertAssetInLibrary($asset, $extension, $library_name, $sub_key, $message = NULL) {
     if (!isset($message)) {
-      $message = sprintf('Asset %s found in library %s', $asset, $library_name);
+      $message = sprintf('Asset %s found in library "%s/%s"', $asset, $extension, $library_name);
     }
     $library = $this->libraryDiscovery->getLibraryByName($extension, $library_name);
     foreach ($library[$sub_key] as $definition) {
@@ -210,7 +231,7 @@ protected function assertAssetInLibrary($asset, $extension, $library_name, $sub_
    */
   protected function assertNoAssetInLibrary($asset, $extension, $library_name, $sub_key, $message = NULL) {
     if (!isset($message)) {
-      $message = sprintf('Asset %s not found in library %s', $asset, $library_name);
+      $message = sprintf('Asset %s not found in library "%s/%s"', $asset, $extension, $library_name);
     }
     $library = $this->libraryDiscovery->getLibraryByName($extension, $library_name);
     foreach ($library[$sub_key] as $definition) {
diff --git a/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml b/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml
index 587070c..3637a0c 100644
--- a/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml
+++ b/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml
@@ -8,5 +8,10 @@ libraries:
 stylesheets-remove:
   - '@theme_test/css/base-remove.css'
 libraries-override:
-  core/drupal.dialog/js/misc/dialog/dialog.js: false
-  core/jquery.farbtastic/css/component/assets/vendor/farbtastic/farbtastic.css: css/farbtastic.css
+  core/drupal.dialog:
+    js:
+      misc/dialog/dialog.js: false
+  core/jquery.farbtastic:
+    css:
+      component:
+        assets/vendor/farbtastic/farbtastic.css: css/farbtastic.css
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 9e12ea1..142470b 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
@@ -23,20 +23,44 @@ libraries-override:
   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
+  # Replace particular library assets.
+  classy/base:
+    css:
+      component:
+        css/components/button.css: css/my-button.css
+        css/components/collapse-processed.css: css/my-collapse-processed.css
+        css/components/container-inline.css: /themes/my_theme/css/my-container-inline.css
+        css/components/details.css: /themes/my_theme/css/my-details.css
+  # Remove particular library assets.
+  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
+  core/jquery:
+    js:
+      assets/vendor/jquery/jquery.min.js: js/collapse.js
   # Use Drupal relative paths.
-  core/drupal.dropbutton/css/component/misc/dropbutton/dropbutton.css: /themes/my_theme/css/dropbutton.css
+  core/drupal.dropbutton:
+    css:
+      component:
+        misc/dropbutton/dropbutton.css: /themes/my_theme/css/dropbutton.css
   # Use stream wrappers.
-  core/drupal.vertical-tabs/css/component/misc/vertical-tabs.css: public://my_css/vertical-tabs.css
+  core/drupal.vertical-tabs:
+    css:
+      component:
+        misc/vertical-tabs.css: public://my_css/vertical-tabs.css
   # Use protocol-relative URI.
-  core/jquery.ui/css/component/assets/vendor/jquery.ui/themes/base/core.css: //my-server/my_theme/css/jquery_ui.css
+  core/jquery.ui:
+    css:
+      component:
+        assets/vendor/jquery.ui/themes/base/core.css: //my-server/my_theme/css/jquery_ui.css
   # Use regular URI.
-  core/jquery.farbtastic/css/component/assets/vendor/farbtastic/farbtastic.css: http://example.com/my_theme/css/farbtastic.css
+  core/jquery.farbtastic:
+    css:
+      component:
+        assets/vendor/farbtastic/farbtastic.css: http://example.com/my_theme/css/farbtastic.css
+
 regions:
   content: Content
   left: Left
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
index e6e6f1f..f55f6d0 100644
--- 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
@@ -7,4 +7,6 @@ core: 8.x
 libraries-override:
   # drupalSettings libraries override. Should throw a
   # \Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException.
-  core/drupal.ajax/drupalSettings/ajaxPageState: { }
+  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
index befe8d8..f31ade0 100644
--- 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
@@ -7,4 +7,5 @@ core: 8.x
 libraries-override:
   # A malformed library asset name. Should throw a
   # \Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException.
-  core/drupal.dialog/css: false
+  core/drupal.dialog:
+    css: false
diff --git a/core/themes/bartik/bartik.info.yml b/core/themes/bartik/bartik.info.yml
index aa0c18d..a74f90c 100644
--- a/core/themes/bartik/bartik.info.yml
+++ b/core/themes/bartik/bartik.info.yml
@@ -7,8 +7,6 @@ version: VERSION
 core: 8.x
 libraries:
   - bartik/global-styling
-libraries-override:
-  classy/base/css/theme/css/layout.css: false
 ckeditor_stylesheets:
   - css/base/elements.css
   - css/components/captions.css
diff --git a/core/themes/seven/seven.info.yml b/core/themes/seven/seven.info.yml
index 1334e99..b983303 100644
--- a/core/themes/seven/seven.info.yml
+++ b/core/themes/seven/seven.info.yml
@@ -9,7 +9,10 @@ core: 8.x
 libraries:
  - seven/global-styling
 libraries-override:
-  core/jquery.ui.dialog/css/component/assets/vendor/jquery.ui/themes/base/dialog.css: false
+  core/jquery.ui.dialog:
+    css:
+      component:
+        assets/vendor/jquery.ui/themes/base/dialog.css: false
 quickedit_stylesheets:
   - css/components/quickedit.css
 regions: