diff -u b/core/lib/Drupal/Core/Theme/ActiveTheme.php b/core/lib/Drupal/Core/Theme/ActiveTheme.php
--- b/core/lib/Drupal/Core/Theme/ActiveTheme.php
+++ b/core/lib/Drupal/Core/Theme/ActiveTheme.php
@@ -81,13 +81,6 @@
   protected $libraries;
 
   /**
-   * The extensions that might be attaching assets.
-   *
-   * @var array
-   */
-  protected $extensions;
-
-  /**
    * Constructs an ActiveTheme object.
    *
    * @param array $values
@@ -105,48 +98,18 @@
       'base_themes' => [],
     ];
 
-    $this->extensions = \Drupal::service('module_handler')->getModuleList();
-    $this->extensions = array_merge($this->extensions, \Drupal::service('theme_handler')->listInfo());
-
     $this->name = $values['name'];
     $this->path = $values['path'];
     $this->engine = $values['engine'];
     $this->owner = $values['owner'];
-    $this->styleSheetsRemove = $this->resolveStyleSheetPlaceholders($values['stylesheets_remove']);
-    $this->styleSheetsOverride = $this->resolveStyleSheetPlaceholders($values['stylesheets_override']);
+    $this->styleSheetsRemove = $values['stylesheets_remove'];
+    $this->styleSheetsOverride = $values['stylesheets_override'];
     $this->libraries = $values['libraries'];
     $this->extension = $values['extension'];
     $this->baseThemes = $values['base_themes'];
   }
 
   /**
-   * Resolves tokens from the stylesheets remove and override.
-   *
-   * @param array $list
-   *
-   * @return array
-   *   List of stylesheets where placeholders are replaced.
-   */
-  public function resolveStyleSheetPlaceholders($list) {
-    foreach ($list as $key => $item) {
-      $token_candidate = explode('/', $key)[0];
-      if (!preg_match('/@[A-z0-9_-]+/', $token_candidate)) {
-        continue;
-      }
-
-      $token = substr($token_candidate, 1);
-
-      if (isset($this->extensions[$token])) {
-        $css_file = str_replace($token_candidate, $this->extensions[$token]->getPath(), $key);
-        $list[$css_file] = $item;
-        unset($list[$key]);
-      }
-    }
-
-    return $list;
-  }
-
-  /**
    * Returns the machine name of the theme.
    *
    * @return string
only in patch2:
unchanged:
--- 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());
+
   }
 
   /**
@@ -162,35 +173,43 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
     foreach ($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 . '/' . $basename;
+        foreach ($base->info['stylesheets-remove'] as $css_file) {
+          $css_file = $this->resolveStyleSheetPlaceholders($css_file);
+          $values['stylesheets_remove'][$css_file] = $css_file;
         }
       }
       if (!empty($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-override'] as $override) {
+          if (is_array($override)) {
+            foreach ($override as $overridden_css => $overriding_css) {
+              $overridden_css = $this->resolveStyleSheetPlaceholders($overridden_css);
+              $values['stylesheets_override'][$overridden_css] = $base_theme_path . '/' . $overriding_css;
+            }
+          }
         }
       }
     }
 
     // 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'][$basename]);
+      foreach ($theme->info['stylesheets-remove'] as $css_file) {
+        $css_file = $this->resolveStyleSheetPlaceholders($css_file);
+        $values['stylesheets_remove'][$css_file] = $css_file;
+        if (isset($values['stylesheets_override'][$css_file])) {
+          unset($values['stylesheets_override'][$css_file]);
         }
       }
     }
     if (!empty($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-override'] as $override) {
+        if (is_array($override)) {
+          foreach ($override as $overridden_css => $overriding_css) {
+            $overridden_css = $this->resolveStyleSheetPlaceholders($overridden_css);
+            $values['stylesheets_override'][$overridden_css] = $theme_path . '/' . $overriding_css;
+          }
+        }
+        if (isset($values['stylesheets_remove'][$overridden_css])) {
+          unset($values['stylesheets_remove'][$overridden_css]);
         }
       }
     }
@@ -228,4 +247,24 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
     return new ActiveTheme($values);
   }
 
+  /**
+   * Resolves tokens from the stylesheets remove and override.
+   *
+   * @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);
+    }
+  }
 }
only in patch2:
unchanged:
--- 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
only in patch2:
unchanged:
--- 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