diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
index 873939c..cb7d9e7 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
@@ -84,7 +84,7 @@ public function getLibraryByName($extension, $name) {
       if (isset($extension[$name]['override'])) {
         $override = $extension[$name]['override'];
         if ($override === FALSE) {
-          // Remove the library definition if false is given.
+          // Remove the library definition if FALSE is given.
           unset($extension[$name]);
           return FALSE;
         }
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index f6de366..89257cd 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -336,32 +336,33 @@ protected function parseLibraryInfo($extension, $path) {
   protected function applyLibrariesOverride($libraries, $extension) {
     $active_theme = $this->themeManager->getActiveTheme();
     $libraries_overrides = $active_theme->getLibrariesOverride();
-    foreach ($libraries as $name => $library) {
+    foreach ($libraries as $library_name => $library) {
       // Process libraries overrides.
-      foreach ($libraries_overrides as $asset => $override) {
+      $namespaced_library_name = "$extension/$library_name";
+      foreach ($libraries_overrides as $definition => $override) {
         // Active theme defines an override for this library.
-        if ($asset === "$extension/$name") {
+        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) {
-            $libraries[$name]['override'] = $override;
+            $libraries[$library_name]['override'] = $override;
           }
           else {
-            $libraries[$name]['override'] = FALSE;
+            $libraries[$library_name]['override'] = FALSE;
           }
         }
-        elseif (strpos($asset, "$extension/$name/") !== FALSE) {
+        elseif ($this->isPartOfLibrary($definition, $namespaced_library_name)) {
           // Active theme defines an override for an asset within this library.
           // Throw an exception if the asset is not properly specified.
-          if (substr_count($asset, '/') < 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".', $asset));
+          if (substr_count($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('/', $asset, 4);
+          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.', $asset));
+            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.
@@ -376,13 +377,13 @@ protected function applyLibrariesOverride($libraries, $extension) {
           // 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[$name], $parents, $key_exists);
+          $attributes = NestedArray::getValue($libraries[$library_name], $parents, $key_exists);
           if ($key_exists) {
             // Remove asset to be overridden, but keep its attributes.
-            NestedArray::unsetValue($libraries[$name], $parents);
+            NestedArray::unsetValue($libraries[$library_name], $parents);
             if ($override) {
               // Replace with an override if specified.
-              NestedArray::setValue($libraries[$name], $new_parents, $attributes);
+              NestedArray::setValue($libraries[$library_name], $new_parents, $attributes);
             }
           }
         }
@@ -413,4 +414,32 @@ protected function isValidUri($string) {
     return count(explode('://', $string)) === 2;
   }
 
+  /**
+   * Checks if the specified definition asset is a full library .
+   *
+   * @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 $library_name.
+   */
+  protected function isEntireLibrary($definition, $library_name) {
+    return $definition === $library_name;
+  }
+
+  /**
+   * Determines if the specified asset definition is a part of a library.
+   *
+   * @param string $definition
+   *   The asset definition being checked.
+   * @param string $library_name
+   *   The name of the library.
+   * @return bool
+   *   Returns true if the definition is part of the library, false otherwise.
+   */
+  protected function isPartOfLibrary($definition, $library_name) {
+    return strpos($definition, "$library_name/") === 0;
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Theme/ActiveTheme.php b/core/lib/Drupal/Core/Theme/ActiveTheme.php
index 6950c76..603dc94 100644
--- a/core/lib/Drupal/Core/Theme/ActiveTheme.php
+++ b/core/lib/Drupal/Core/Theme/ActiveTheme.php
@@ -224,7 +224,7 @@ public function getRegions() {
    * @endcode
    *
    * @return array
-   *   The list of libraries-override.
+   *   The list of libraries overrides.
    */
   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 0d0af89..8a240d5 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -164,12 +164,12 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
     // @todo Remove in Drupal 9.0.x.
     $values['stylesheets_remove'] = $this->prepareStylesheetsRemove($theme, $base_themes);
 
-    // Prepare libraries overrides from this theme and ancestor themes.
-    // This allows child themes to easily remove CSS files from base themes and
+    // 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'] = [];
 
-    // Get libraries-override declared by base theme.
+    // Get libraries overrides declared by base themes.
     foreach ($base_themes as $base) {
       if (!empty($base->info['libraries-override'])) {
         foreach ($base->info['libraries-override'] as $library => $override) {
@@ -178,7 +178,7 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
       }
     }
 
-    // Add libraries-override declared by this theme.
+    // 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;
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 40a52a9..9e12ea1 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
@@ -31,9 +31,9 @@ libraries-override:
   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
-  # Use streamwrappers.
+  # Use stream wrappers.
   core/drupal.vertical-tabs/css/component/misc/vertical-tabs.css: public://my_css/vertical-tabs.css
-  # Use protocol-free URI.
+  # 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
   # Use regular URI.
   core/jquery.farbtastic/css/component/assets/vendor/farbtastic/farbtastic.css: http://example.com/my_theme/css/farbtastic.css