diff --git a/core/lib/Drupal/Core/Asset/AssetResolver.php b/core/lib/Drupal/Core/Asset/AssetResolver.php
index c41e14e..0d679b8 100644
--- a/core/lib/Drupal/Core/Asset/AssetResolver.php
+++ b/core/lib/Drupal/Core/Asset/AssetResolver.php
@@ -123,23 +123,17 @@ public function getCssAssets(AttachedAssetsInterface $assets, $optimize) {
           // order.
           $options['weight'] += count($css) / 1000;
 
-          // Add the data to the CSS array depending on the type.
+          // Depending on the type, some options may need processing.
           switch ($options['type']) {
             case 'file':
-              // Local CSS files are keyed by basename; if a file with the same
-              // basename is added more than once, it gets overridden.
-              // By default, take over the filename as basename.
+              // Local CSS files need the basename to be saved because it is
+              // used for overriding and removing stylesheets.
               if (!isset($options['basename'])) {
                 $options['basename'] = drupal_basename($options['data']);
               }
-              $css[$options['basename']] = $options;
               break;
-
-            default:
-              // External files are keyed by their full URI, so the same CSS
-              // file is not added twice.
-              $css[$options['data']] = $options;
           }
+          $css[$options['data']] = $options;
         }
       }
     }
@@ -151,22 +145,14 @@ public function getCssAssets(AttachedAssetsInterface $assets, $optimize) {
     // Sort CSS items, so that they appear in the correct order.
     uasort($css, 'static::sort');
 
-    // Allow themes to remove CSS files by basename.
+    // Allow themes to remove CSS files by CSS file location.
     if ($stylesheet_remove = $theme_info->getStyleSheetsRemove()) {
       foreach ($css as $key => $options) {
-        if (isset($options['basename']) && isset($stylesheet_remove[$options['basename']])) {
+        if (isset($stylesheet_remove[$key])) {
           unset($css[$key]);
         }
       }
     }
-    // Allow themes to conditionally override CSS files by basename.
-    if ($stylesheet_override = $theme_info->getStyleSheetsOverride()) {
-      foreach ($css as $key => $options) {
-        if (isset($options['basename']) && isset($stylesheet_override[$options['basename']])) {
-          $css[$key]['data'] = $stylesheet_override[$options['basename']];
-        }
-      }
-    }
 
     if ($optimize) {
       $css = \Drupal::service('asset.css.collection_optimizer')->optimize($css);
diff --git a/core/lib/Drupal/Core/Theme/ActiveTheme.php b/core/lib/Drupal/Core/Theme/ActiveTheme.php
index e1d9a96..aace6f9 100644
--- a/core/lib/Drupal/Core/Theme/ActiveTheme.php
+++ b/core/lib/Drupal/Core/Theme/ActiveTheme.php
@@ -67,13 +67,6 @@ class ActiveTheme {
   protected $styleSheetsRemove;
 
   /**
-   * The stylesheets which are overridden by the theme.
-   *
-   * @var array
-   */
-  protected $styleSheetsOverride;
-
-  /**
    * The libraries provided by the theme.
    *
    * @var array
@@ -92,17 +85,16 @@ public function __construct(array $values) {
       'engine' => 'twig',
       'owner' => 'twig',
       'stylesheets_remove' => [],
-      'stylesheets_override' => [],
       'libraries' => [],
       'extension' => 'html.twig',
       'base_themes' => [],
     ];
+
     $this->name = $values['name'];
     $this->path = $values['path'];
     $this->engine = $values['engine'];
     $this->owner = $values['owner'];
     $this->styleSheetsRemove = $values['stylesheets_remove'];
-    $this->styleSheetsOverride = $values['stylesheets_override'];
     $this->libraries = $values['libraries'];
     $this->extension = $values['extension'];
     $this->baseThemes = $values['base_themes'];
@@ -165,15 +157,6 @@ public function getLibraries() {
   }
 
   /**
-   * Returns the overridden stylesheets by the theme.
-   *
-   * @return mixed
-   */
-  public function getStyleSheetsOverride() {
-    return $this->styleSheetsOverride;
-  }
-
-  /**
    * Returns the removed stylesheets by the theme.
    *
    * @return mixed
diff --git a/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
index 29c8205..70bcbf8 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -38,6 +38,13 @@ class ThemeInitialization implements ThemeInitializationInterface {
   protected $root;
 
   /**
+   * The extensions that might be attaching assets.
+   *
+   * @var array
+   */
+  protected $extensions;
+
+  /**
    * Constructs a new ThemeInitialization object.
    *
    * @param string $root
@@ -51,6 +58,10 @@ public function __construct($root, ThemeHandlerInterface $theme_handler, StateIn
     $this->root = $root;
     $this->themeHandler = $theme_handler;
     $this->state = $state;
+
+    $this->extensions = \Drupal::service('module_handler')->getModuleList();
+    $this->extensions = array_merge($this->extensions,  $this->themeHandler->listInfo());
+
   }
 
   /**
@@ -151,47 +162,26 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
     $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 override parent
-    // theme stylesheets easily.
-    // CSS file basenames to override, pointing to the final, overridden filepath.
-    $values['stylesheets_override'] = array();
-    // CSS file basenames to remove.
+    // We work it this way so that we can have child themes remove CSS files
+    // easily from parent.
     $values['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 $basename) {
-          $values['stylesheets_remove'][$basename] = $base_theme_path . '/' . $basename;
-        }
-      }
-      if (!empty($base->info['stylesheets-override'])) {
-        foreach ($base->info['stylesheets-override'] as $name) {
-          $basename = drupal_basename($name);
-          $values['stylesheets_override'][$basename] = $base_theme_path . '/' . $name;
+        foreach ($base->info['stylesheets-remove'] as $css_file) {
+          $css_file = $this->resolveStyleSheetPlaceholders($css_file);
+          $values['stylesheets_remove'][$css_file] = $css_file;
         }
       }
     }
 
     // Add stylesheets used by this theme.
     if (!empty($theme->info['stylesheets-remove'])) {
-      foreach ($theme->info['stylesheets-remove'] as $basename) {
-        $values['stylesheets_remove'][$basename] = $theme_path . '/' . $basename;
-
-        if (isset($values['stylesheets_override'][$basename])) {
-          unset($values['stylesheets_override'][$basename]);
-        }
-      }
-    }
-    if (!empty($theme->info['stylesheets-override'])) {
-      foreach ($theme->info['stylesheets-override'] as $name) {
-        $basename = drupal_basename($name);
-        $values['stylesheets_override'][$basename] = $theme_path . '/' . $name;
-
-        if (isset($values['stylesheets_remove'][$basename])) {
-          unset($values['stylesheets_remove'][$basename]);
-        }
+      foreach ($theme->info['stylesheets-remove'] as $css_file) {
+        $css_file = $this->resolveStyleSheetPlaceholders($css_file);
+        $values['stylesheets_remove'][$css_file] = $css_file;
       }
     }
 
@@ -228,4 +218,24 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
     return new ActiveTheme($values);
   }
 
+  /**
+   * Resolves tokens from the stylesheets remove.
+   *
+   * @param array $list
+   *
+   * @return array
+   *   List of stylesheets where placeholders are replaced.
+   */
+  public function resolveStyleSheetPlaceholders($css_file) {
+    $token_candidate = explode('/', $css_file)[0];
+    if (!preg_match('/@[A-z0-9_-]+/', $token_candidate)) {
+      return $css_file;
+    }
+
+    $token = substr($token_candidate, 1);
+
+    if (isset($this->extensions[$token])) {
+      return str_replace($token_candidate, $this->extensions[$token]->getPath(), $css_file);
+    }
+  }
 }
diff --git a/core/modules/system/src/Tests/Common/AttachedAssetsTest.php b/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
index a52db9b..d111f44 100644
--- a/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
+++ b/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
@@ -89,7 +89,7 @@ function testAddFiles() {
 
     $css = $this->assetResolver->getCssAssets($assets, FALSE);
     $js = $this->assetResolver->getJsAssets($assets, FALSE)[1];
-    $this->assertTrue(array_key_exists('bar.css', $css), 'CSS files are correctly added.');
+    $this->assertTrue(array_key_exists('core/modules/system/tests/modules/common_test/bar.css', $css), 'CSS files are correctly added.');
     $this->assertTrue(array_key_exists('core/modules/system/tests/modules/common_test/foo.js', $js), 'JavaScript files are correctly added.');
 
     $css_render_array = \Drupal::service('asset.css.collection_renderer')->render($css);
@@ -470,7 +470,7 @@ function testAddJsFileWithQueryString() {
 
     $css = $this->assetResolver->getCssAssets($assets, FALSE);
     $js = $this->assetResolver->getJsAssets($assets, FALSE)[1];
-    $this->assertTrue(array_key_exists('querystring.css?arg1=value1&arg2=value2', $css), 'CSS file with query string is correctly added.');
+    $this->assertTrue(array_key_exists('core/modules/system/tests/modules/common_test/querystring.css?arg1=value1&arg2=value2', $css), 'CSS file with query string is correctly added.');
     $this->assertTrue(array_key_exists('core/modules/system/tests/modules/common_test/querystring.js?arg1=value1&arg2=value2', $js), 'JavaScript file with query string is correctly added.');
 
     $css_render_array = \Drupal::service('asset.css.collection_renderer')->render($css);
diff --git a/core/modules/system/src/Tests/Theme/ThemeInfoTest.php b/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
index aeb8846..fc42a5b 100644
--- a/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
@@ -86,6 +86,10 @@ function testStylesheets() {
     $this->assertIdentical(0, count($this->xpath("//link[contains(@href, 'sub-remove.css')]")), "sub-remove.css not found");
     $this->assertIdentical(0, count($this->xpath("//link[contains(@href, 'base-add.sub-remove.css')]")), "base-add.sub-remove.css not found");
     $this->assertIdentical(0, count($this->xpath("//link[contains(@href, 'base-override.sub-remove.css')]")), "base-override.sub-remove.css not found");
+
+    // Verify that CSS files with the same name are loaded from both the base theme and subtheme.
+    $this->assertIdentical(1, count($this->xpath("//link[contains(@href, '$base/samename.css')]")), "$base/samename.css found");
+    $this->assertIdentical(1, count($this->xpath("//link[contains(@href, '$sub/samename.css')]")), "$sub/samename.css found");
   }
 
   /**
diff --git a/core/modules/system/tests/themes/test_basetheme/.test_basetheme.info.yml.swo b/core/modules/system/tests/themes/test_basetheme/.test_basetheme.info.yml.swo
new file mode 100644
index 0000000..c06a858
Binary files /dev/null and b/core/modules/system/tests/themes/test_basetheme/.test_basetheme.info.yml.swo differ
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 1c07b3f..dcf03eb 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
@@ -6,8 +6,8 @@ core: 8.x
 libraries:
   - test_basetheme/global-styling
 stylesheets-override:
-  - base-override.css
-  - base-override.sub-remove.css
+  - @theme_test/css/base-override.css: base-override.css
+  - @theme_test/css/base-override.sub-remove.css: base-override.sub-remove.css
 stylesheets-remove:
-  - base-remove.css
-  - base-remove.sub-override.css
+  - @theme_test/css/base-remove.css
+  - @theme_test/css/base-remove.sub-override.css
diff --git a/core/modules/system/tests/themes/test_basetheme/test_basetheme.libraries.yml b/core/modules/system/tests/themes/test_basetheme/test_basetheme.libraries.yml
index 1c72d30..49e4ba1 100644
--- a/core/modules/system/tests/themes/test_basetheme/test_basetheme.libraries.yml
+++ b/core/modules/system/tests/themes/test_basetheme/test_basetheme.libraries.yml
@@ -5,3 +5,4 @@ global-styling:
       base-add.css: {}
       base-add.sub-override.css: {}
       base-add.sub-remove.css: {}
+      samename.css: {}
diff --git a/core/modules/system/tests/themes/test_subtheme/.test_subtheme.info.yml.swp b/core/modules/system/tests/themes/test_subtheme/.test_subtheme.info.yml.swp
new file mode 100644
index 0000000..7a3517c
Binary files /dev/null and b/core/modules/system/tests/themes/test_subtheme/.test_subtheme.info.yml.swp differ
diff --git a/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml b/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml
index b15efa9..1d1a19d 100644
--- a/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml
+++ b/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml
@@ -7,10 +7,10 @@ base theme: test_basetheme
 libraries:
   - test_subtheme/global-styling
 stylesheets-override:
-  - css/sub-override.css
-  - css/base-add.sub-override.css
-  - css/base-remove.sub-override.css
+  - @theme_test/css/sub-override.css: css/sub-override.css
+  - @theme_test/css/base-remove.sub-override.css: css/base-remove.sub-override.css
+  - @test_basetheme/base-add.sub-override.css: css/base-add.sub-override.css
 stylesheets-remove:
-  - sub-remove.css
-  - base-add.sub-remove.css
-  - base-override.sub-remove.css
+  - @theme_test/css/sub-remove.css
+  - @test_basetheme/base-add.sub-remove.css
+  - @test_basetheme/base-override.sub-remove.css
diff --git a/core/modules/system/tests/themes/test_subtheme/test_subtheme.libraries.yml b/core/modules/system/tests/themes/test_subtheme/test_subtheme.libraries.yml
index efce7ae..931dffe 100644
--- a/core/modules/system/tests/themes/test_subtheme/test_subtheme.libraries.yml
+++ b/core/modules/system/tests/themes/test_subtheme/test_subtheme.libraries.yml
@@ -3,3 +3,4 @@ global-styling:
   css:
     base:
       css/sub-add.css: {}
+      css/samename.css: {}
diff --git a/core/themes/seven/.seven.info.yml.swo b/core/themes/seven/.seven.info.yml.swo
new file mode 100644
index 0000000..3fc87eb
Binary files /dev/null and b/core/themes/seven/.seven.info.yml.swo differ
diff --git a/core/themes/seven/seven.theme b/core/themes/seven/seven.theme
index c24035e..b4075cb 100644
--- a/core/themes/seven/seven.theme
+++ b/core/themes/seven/seven.theme
@@ -197,3 +197,6 @@ function seven_form_node_form_alter(&$form, FormStateInterface $form_state) {
   $form['revision_information']['#type'] = 'container';
   $form['revision_information']['#group'] = 'meta';
 }
+
+function seven_library_alter() {
+}