diff --git a/core/includes/common.inc b/core/includes/common.inc
index edd7c2d..0bc786c 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -2601,9 +2601,20 @@ function drupal_add_css($data = NULL, $options = NULL) {
         // key as $data could be a very long string of CSS.
         $css[] = $options;
         break;
+
+      case 'file':
+        // Local CSS files are keyed by basename; if a file with the same
+        // basename is added more than once, it gets overridden.
+        // By default, take over the filename as basename.
+        if (!isset($options['basename'])) {
+          $options['basename'] = drupal_basename($data);
+        }
+        $css[$options['basename']] = $options;
+        break;
+
       default:
-        // Local and external files must keep their name as the associative key
-        // so the same CSS file is not be added twice.
+        // External files are keyed by their full URI, so the same CSS file is
+        // not added twice.
         $css[$data] = $options;
     }
   }
@@ -2641,6 +2652,8 @@ function drupal_add_css($data = NULL, $options = NULL) {
  * @see drupal_add_css()
  */
 function drupal_get_css($css = NULL, $skip_alter = FALSE) {
+  global $theme_info;
+
   if (!isset($css)) {
     $css = drupal_add_css();
   }
@@ -2653,17 +2666,12 @@ function drupal_get_css($css = NULL, $skip_alter = FALSE) {
   // Sort CSS items, so that they appear in the correct order.
   uasort($css, 'drupal_sort_css_js');
 
-  // Remove the overridden CSS files. Later CSS files override former ones.
-  $previous_item = array();
-  foreach ($css as $key => $item) {
-    if ($item['type'] == 'file') {
-      // If defined, force a unique basename for this file.
-      $basename = isset($item['basename']) ? $item['basename'] : drupal_basename($item['data']);
-      if (isset($previous_item[$basename])) {
-        // Remove the previous item that shared the same base name.
-        unset($css[$previous_item[$basename]]);
+  // Allow themes to conditionally override module CSS files by basename.
+  if (!empty($theme_info->stylesheets_override)) {
+    foreach ($css as $key => $options) {
+      if (isset($options['basename']) && isset($theme_info->stylesheets_override[$options['basename']])) {
+        $css[$key]['data'] = $theme_info->stylesheets_override[$options['basename']];
       }
-      $previous_item[$basename] = $key;
     }
   }
 
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 3ce399b..e38739e 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -149,6 +149,10 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
   // We work it this way so that we can have child themes override parent
   // theme stylesheets easily.
   $final_stylesheets = array();
+  // List of CSS file basenames to override, pointing to the theme name that
+  // holds the final override.
+  // @todo Try to prepare this in _system_rebuild_theme_data() already.
+  $theme->stylesheets_override = array();
 
   // Grab stylesheets from base theme
   foreach ($base_theme as $base) {
@@ -159,6 +163,12 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
         }
       }
     }
+    if (!empty($base->info['stylesheets-override'])) {
+      $base_theme_path = dirname($base->filename);
+      foreach ($base->info['stylesheets-override'] as $basename) {
+        $theme->stylesheets_override[$basename] = $base_theme_path . '/' . $basename;
+      }
+    }
   }
 
   // Add stylesheets used by this theme.
@@ -169,6 +179,11 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
       }
     }
   }
+  if (!empty($theme->info['stylesheets-override'])) {
+    foreach ($theme->info['stylesheets-override'] as $basename) {
+      $theme->stylesheets_override[$basename] = $theme_path . '/' . $basename;
+    }
+  }
 
   // And now add the stylesheets properly
   foreach ($final_stylesheets as $media => $stylesheets) {
diff --git a/core/modules/color/color.module b/core/modules/color/color.module
index 56c04cf..680ceed 100644
--- a/core/modules/color/color.module
+++ b/core/modules/color/color.module
@@ -77,7 +77,7 @@ function _color_html_alter(&$vars) {
         if (drupal_basename($old_path) == drupal_basename($color_path)) {
           // Replace the path to the new css file.
           // This keeps the order of the stylesheets intact.
-          $vars['css'][$old_path]['data'] = $color_path;
+          $vars['css'][drupal_basename($old_path)]['data'] = $color_path;
         }
       }
     }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Ajax/FrameworkTest.php b/core/modules/system/lib/Drupal/system/Tests/Ajax/FrameworkTest.php
index 00e7f14..e7aa0e0 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Ajax/FrameworkTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Ajax/FrameworkTest.php
@@ -78,8 +78,11 @@ function testLazyLoad() {
       'css' => drupal_get_path('module', 'system') . '/system.admin.css',
       'js' => drupal_get_path('module', 'system') . '/system.js',
     );
+    // CSS files are stored by basename, see drupal_add_css().
+    $expected_css_basename = drupal_basename($expected['css']);
+
     // @todo D8: Add a drupal_css_defaults() helper function.
-    $expected_css_html = drupal_get_css(array($expected['css'] => array(
+    $expected_css_html = drupal_get_css(array($expected_css_basename => array(
       'type' => 'file',
       'group' => CSS_DEFAULT,
       'weight' => 0,
@@ -135,7 +138,7 @@ function testLazyLoad() {
 
     // Verify the expected CSS file was added, both to Drupal.settings, and as
     // an Ajax command for inclusion into the HTML.
-    $this->assertEqual($new_css, $original_css + array($expected['css'] => 1), format_string('Page state now has the %css file.', array('%css' => $expected['css'])));
+    $this->assertEqual($new_css, $original_css + array($expected_css_basename => 1), format_string('Page state now has the %css file.', array('%css' => $expected['css'])));
     $this->assertCommand($commands, array('data' => $expected_css_html), format_string('Page now has the %css file.', array('%css' => $expected['css'])));
 
     // Verify the expected JS file was added, both to Drupal.settings, and as
diff --git a/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsTest.php b/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsTest.php
index 9009bbb..8e89935 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsTest.php
@@ -69,7 +69,7 @@ function testModuleInfo() {
   function testAddFile() {
     $path = drupal_get_path('module', 'simpletest') . '/simpletest.css';
     $css = drupal_add_css($path);
-    $this->assertEqual($css[$path]['data'], $path, 'Adding a CSS file caches it properly.');
+    $this->assertEqual($css['simpletest.css']['data'], $path);
   }
 
   /**
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.info b/core/modules/system/tests/themes/test_theme/test_theme.info
index b5d1bfc..53a7624 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.info
+++ b/core/modules/system/tests/themes/test_theme/test_theme.info
@@ -3,16 +3,6 @@ description = Theme for testing the theme system
 core = 8.x
 hidden = TRUE
 
-; Normally, themes may list CSS files like this, and if they exist in the theme
-; folder, then they get added to the page. If they have the same file name as a
-; module CSS file, then the theme's version overrides the module's version, so
-; that the module's version is not added to the page. Additionally, a theme may
-; have an entry like this one, without having the corresponding CSS file in the
-; theme's folder, and in this case, it just stops the module's version from
-; being loaded, and does not replace it with an alternate version. We have this
-; here in order for a test to ensure that this correctly prevents the module
-; version from being loaded, and that errors aren't caused by the lack of this
-; file within the theme folder.
-stylesheets[all][] = system.base.css
+stylesheets-override[] = system.base.css
 
 settings[theme_test_setting] = default value
diff --git a/core/modules/views/lib/Drupal/views/Tests/Plugin/CacheTest.php b/core/modules/views/lib/Drupal/views/Tests/Plugin/CacheTest.php
index 58bf751..a04ded8 100644
--- a/core/modules/views/lib/Drupal/views/Tests/Plugin/CacheTest.php
+++ b/core/modules/views/lib/Drupal/views/Tests/Plugin/CacheTest.php
@@ -187,7 +187,7 @@ function testHeaderStorage() {
     $js_path = drupal_get_path('module', 'views_test_data') . '/views_cache.test.js';
     $js = drupal_add_js();
 
-    $this->assertTrue(isset($css[$css_path]), 'Make sure the css is added for cached views.');
+    $this->assertTrue(isset($css[basename($css_path)]), 'Make sure the css is added for cached views.');
     $this->assertTrue(isset($js[$js_path]), 'Make sure the js is added for cached views.');
     $this->assertFalse(!empty($view->build_info['pre_render_called']), 'Make sure hook_views_pre_render is not called for the cached view.');
 
@@ -211,7 +211,7 @@ function testHeaderStorage() {
     $css = drupal_add_css();
     $js = drupal_add_js();
 
-    $this->assertFalse(isset($css[$system_css_path]), 'Make sure that unrelated css is not added.');
+    $this->assertFalse(isset($css[basename($system_css_path)]), 'Make sure that unrelated css is not added.');
     $this->assertFalse(isset($js[$system_js_path]), 'Make sure that unrelated js is not added.');
   }
 
diff --git a/core/themes/seven/seven.info b/core/themes/seven/seven.info
index 87edba3..b737eaf 100644
--- a/core/themes/seven/seven.info
+++ b/core/themes/seven/seven.info
@@ -3,8 +3,14 @@ description = A simple one-column, tableless, fluid width administration theme.
 package = Core
 version = VERSION
 core = 8.x
+
 stylesheets[screen][] = style.css
+stylesheets-override[] = vertical-tabs.css
+stylesheets-override[] = vertical-tabs-rtl.css
+stylesheets-override[] = jquery.ui.theme.css
+
 settings[shortcut_module_link] = 1
+
 regions[content] = Content
 regions[help] = Help
 regions[page_top] = Page top
diff --git a/core/themes/seven/template.php b/core/themes/seven/template.php
index b92d064..335f18e 100644
--- a/core/themes/seven/template.php
+++ b/core/themes/seven/template.php
@@ -104,20 +104,3 @@ function seven_tablesort_indicator($variables) {
     return theme('image', array('uri' => $theme_path . '/images/arrow-desc.png', 'alt' => t('sort descending'), 'width' => 13, 'height' => 13, 'title' => t('sort descending')));
   }
 }
-
-/**
- * Implements hook_css_alter().
- */
-function seven_css_alter(&$css) {
-  // Use Seven's vertical tabs style instead of the default one.
-  if (isset($css['core/misc/vertical-tabs.css'])) {
-    $css['core/misc/vertical-tabs.css']['data'] = drupal_get_path('theme', 'seven') . '/vertical-tabs.css';
-  }
-  if (isset($css['core/misc/vertical-tabs-rtl.css'])) {
-    $css['core/misc/vertical-tabs-rtl.css']['data'] = drupal_get_path('theme', 'seven') . '/vertical-tabs-rtl.css';
-  }
-  // Use Seven's jQuery UI theme style instead of the default one.
-  if (isset($css['core/misc/ui/themes/base/jquery.ui.theme.css'])) {
-    $css['core/misc/ui/themes/base/jquery.ui.theme.css']['data'] = drupal_get_path('theme', 'seven') . '/jquery.ui.theme.css';
-  }
-}