Split Seven's style.css into SMACSS categories

Adding Needs tests tag, since the tests require an update.

Updated the summary.

Looks good to me. Would love to get this in to start documenting the css.

+++ b/core/modules/system/src/Tests/Menu/MenuRouterTest.php
@@ -266,7 +266,7 @@ public function testThemeIntegration() {
-    $this->assertRaw('seven/css/style.css', "The administrative theme's CSS appears on the page.");
+    $this->assertRaw('seven/css/base/elements.css', "The administrative theme's CSS appears on the page.");

@@ -285,7 +285,7 @@ protected function doTestThemeCallbackMaintenanceMode() {
-    $this->assertRaw('seven/css/style.css', "The administrative theme's CSS appears on the page.");
+    $this->assertRaw('seven/css/base/elements.css', "The administrative theme's CSS appears on the page.");

+++ b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
@@ -166,13 +166,36 @@ public function testRebuildThemeData() {
-        'css/seven.base.css' => DRUPAL_ROOT . '/core/themes/seven/css/seven.base.css',
-        'css/style.css' => DRUPAL_ROOT . '/core/themes/seven/css/style.css',
-        'css/layout.css' => DRUPAL_ROOT . '/core/themes/seven/css/layout.css',
+        'css/base/elements.css' => DRUPAL_ROOT . '/core/themes/seven/css/base/elements.css',

Interesting that our underlying system tests test stuff in Seven theme, but that's a pre-existing condition. :)

I looked through this pretty carefully and it all seems to be affecting things only in Seven theme, not in any other CSS in core, so should be safe. It's also been RTBC for about a week, which should be ample time for people to raise concerns if there are any.

Committed and pushed to 8.x. Thanks!

Sorry it added a CSS regression.

D8 now

D8 before

it is fine with aggregate but not without it.

Aggregate on

Aggregate off

NVM it was ABP of my chrome which doesn't allow me to load branding.css see http://stackoverflow.com/questions/23341765/getting-neterr-blocked-by-cl...
It is working fine in firefox and after disabling ABP in chrome.
Sorry for the noise and thank you for your patience @LewisNyman