diff --git a/core/lib/Drupal/Core/Theme/ActiveTheme.php b/core/lib/Drupal/Core/Theme/ActiveTheme.php
index 30e70be..6950c76 100644
--- a/core/lib/Drupal/Core/Theme/ActiveTheme.php
+++ b/core/lib/Drupal/Core/Theme/ActiveTheme.php
@@ -224,6 +224,7 @@ public function getRegions() {
    * @endcode
    *
    * @return array
+   *   The list of libraries-override.
    */
   public function getLibrariesOverride() {
     return $this->librariesOverride;
diff --git a/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
index f3ed6ef..52af336 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -268,6 +268,7 @@ protected function resolveStyleSheetPlaceholders($css_file) {
    *   An array of base themes.
    *
    * @return string[]
+   *   The list of stylesheets-remove specified in the *.info.yml file.
    *
    * @todo Remove in Drupal 9.0.x
    */
@@ -300,6 +301,9 @@ protected function prepareStylesheetsRemove(Extension $theme, $base_themes) {
   /**
    * 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
@@ -308,6 +312,7 @@ protected function prepareStylesheetsRemove(Extension $theme, $base_themes) {
    *   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.
diff --git a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
index dbd0b3e..49bf172 100644
--- a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
+++ b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
@@ -141,7 +141,7 @@ public function testLibrariesOverrideMalformedAsset() {
   }
 
   /**
-   * Tests library assets with edge-casey paths.
+   * Tests library assets with other ways for specifying paths.
    */
   public function testLibrariesOverrideOtherAssetLibraryNames() {
     $this->container->get('theme_handler')->install(['test_theme']);
@@ -161,7 +161,7 @@ public function testLibrariesOverrideOtherAssetLibraryNames() {
     $library = $library_discovery->getLibraryByName('core', 'drupal.dropbutton');
     $this->assertAssetInLibrary($library['css'], 'drupal.dropbutton', 'themes/my_theme/css/dropbutton.css');
 
-    // Assert streamwrappers.
+    // Assert streamwrapper paths.
     $library = $library_discovery->getLibraryByName('core', 'drupal.vertical-tabs');
     $this->assertAssetInLibrary($library['css'], 'drupal.vertical-tabs', 'public://my_css/vertical-tabs.css');
 
@@ -213,6 +213,7 @@ public function testBaseThemeLibrariesOverrideInSubTheme() {
    *  (optional) A message to display with the assertion.
    *
    * @return bool
+   *  TRUE if the specified asset is found in the library.
    */
   protected function assertAssetInLibrary($library_item, $library_name, $asset, $message = NULL) {
     if (!isset($message)) {
@@ -239,6 +240,7 @@ protected function assertAssetInLibrary($library_item, $library_name, $asset, $m
    *   (optional) A message to display with the assertion.
    *
    * @return bool
+   *  TRUE if the specified asset is not found in the library.
    */
   protected function assertNoAssetInLibrary($library_item, $library_name, $asset, $message = NULL) {
     if (!isset($message)) {