diff --git a/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
index 0dbd0c6..7fb7f23 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -99,7 +99,7 @@ public function getActiveThemeByName($theme_name) {
     $ancestor = $theme_name;
     while ($ancestor && isset($themes[$ancestor]->base_theme)) {
       $ancestor = $themes[$ancestor]->base_theme;
-      $base_themes[] = $themes[$ancestor];
+      array_unshift($base_themes, $themes[$ancestor]);
     }
 
     $active_theme = $this->getActiveTheme($themes[$theme_name], $base_themes);
diff --git a/core/modules/system/src/Tests/Theme/ThemeTest.php b/core/modules/system/src/Tests/Theme/ThemeTest.php
index 4573d01..980270b 100644
--- a/core/modules/system/src/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeTest.php
@@ -154,7 +154,8 @@ function testFrontPageThemeSuggestion() {
   }
 
   /**
-   * Ensures a theme's .info.yml file is able to override a module CSS file from being added to the page.
+   * Ensures a theme's .info.yml and .libraries.yml files are able to override
+   * a module CSS file from being added to the page.
    *
    * @see test_theme.info.yml
    */
@@ -169,6 +170,13 @@ function testCSSOverride() {
     $this->drupalGet('theme-test/suggestion');
     $this->assertNoText('system.module.css', 'The theme\'s .info.yml file is able to override a module CSS file from being added to the page.');
 
+    // Ensure that parent's layout.css file gets overridden by test theme's
+    // layout.css file.
+    $classy_layout_css_url = drupal_get_path('theme', 'classy') .'/css/layout.css';
+    $this->assertIdentical(0, count($this->xpath("//link[contains(@href, '" . $classy_layout_css_url . "')]")), $classy_layout_css_url . " not found");
+    $theme_test_layout_css_url = drupal_get_path('theme', 'test_theme') .'/css/layout.css';
+    $this->assertIdentical(1, count($this->xpath("//link[contains(@href, '" . $theme_test_layout_css_url . "')]")), $theme_test_layout_css_url . " found");
+
     // Also test with aggregation enabled, simply ensuring no PHP errors are
     // triggered during drupal_build_css_cache() when a source file doesn't
     // exist. Then allow remaining tests to continue with aggregation disabled
diff --git a/core/modules/system/tests/themes/test_theme/css/layout.css b/core/modules/system/tests/themes/test_theme/css/layout.css
new file mode 100644
index 0000000..12b06dc
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/css/layout.css
@@ -0,0 +1,6 @@
+
+/**
+ * This file is for testing CSS file override through *.libraries.yml file in
+ * ThemeTest::testCSSOverride().
+ * No contents are necessary.
+ */
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 9eebc90..5000e74 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
@@ -14,6 +14,8 @@ description: 'Theme for testing the theme system'
 version: VERSION
 base theme: classy
 core: 8.x
+libraries:
+  - test_theme/global-styling
 stylesheets-remove:
   - system.module.css
 regions:
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml b/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml
new file mode 100644
index 0000000..ed21a2d
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml
@@ -0,0 +1,5 @@
+global-styling:
+  version: VERSION
+  css:
+    theme:
+      css/layout.css: {}