diff -u b/core/modules/system/src/Tests/Theme/ThemeInfoTest.php b/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
--- b/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
@@ -74,18 +74,11 @@
     $this->drupalGet('theme-test/info/stylesheets');
 
     $this->assertIdentical(1, count($this->xpath("//link[contains(@href, '$base/base-add.css')]")), "$base/base-add.css found");
-    $this->assertIdentical(1, count($this->xpath("//link[contains(@href, '$base/base-override.css')]")), "$base/base-override.css found");
     $this->assertIdentical(0, count($this->xpath("//link[contains(@href, 'base-remove.css')]")), "base-remove.css not found");
 
     $this->assertIdentical(1, count($this->xpath("//link[contains(@href, '$sub/sub-add.css')]")), "$sub/sub-add.css found");
-
-    $this->assertIdentical(1, count($this->xpath("//link[contains(@href, '$sub/sub-override.css')]")), "$sub/sub-override.css found");
-    $this->assertIdentical(1, count($this->xpath("//link[contains(@href, '$sub/base-add.sub-override.css')]")), "$sub/base-add.sub-override.css found");
-    $this->assertIdentical(1, count($this->xpath("//link[contains(@href, '$sub/base-remove.sub-override.css')]")), "$sub/base-remove.sub-override.css found");
-
     $this->assertIdentical(0, count($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");
diff -u b/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml b/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml
--- b/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml
+++ b/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml
@@ -8,6 +8,2 @@
-stylesheets-override:
-  - @theme_test/css/base-override.css: base-override.css
-  - @theme_test/css/base-override.sub-remove.css: base-override.sub-remove.css
 stylesheets-remove:
   - @theme_test/css/base-remove.css
-  - @theme_test/css/base-remove.sub-override.css
diff -u b/core/modules/system/tests/themes/test_basetheme/test_basetheme.libraries.yml b/core/modules/system/tests/themes/test_basetheme/test_basetheme.libraries.yml
--- b/core/modules/system/tests/themes/test_basetheme/test_basetheme.libraries.yml
+++ b/core/modules/system/tests/themes/test_basetheme/test_basetheme.libraries.yml
@@ -4,5 +4,4 @@
     base:
       base-add.css: {}
-      base-add.sub-override.css: {}
       base-add.sub-remove.css: {}
       samename.css: {}
diff -u b/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml b/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml
--- b/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml
+++ b/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml
@@ -9,8 +9,3 @@
-stylesheets-override:
-  - @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:
   - @theme_test/css/sub-remove.css
   - @test_basetheme/base-add.sub-remove.css
-  - @test_basetheme/base-override.sub-remove.css
reverted:
--- b/core/themes/seven/seven.theme
+++ a/core/themes/seven/seven.theme
@@ -197,6 +197,3 @@
   $form['revision_information']['#type'] = 'container';
   $form['revision_information']['#group'] = 'meta';
 }
-
-function seven_library_alter() {
-}