diff --git a/core/includes/update.inc b/core/includes/update.inc
index acdf83e..44d96a4 100644
--- a/core/includes/update.inc
+++ b/core/includes/update.inc
@@ -12,6 +12,7 @@
 use Drupal\Core\Config\FileStorage;
 use Drupal\Core\Config\ConfigException;
 use Drupal\Component\Uuid\Uuid;
+use Drupal\Component\Utility\NestedArray;
 
 /**
  * Minimum schema version of Drupal 7 required for upgrade to Drupal 8.
@@ -1202,18 +1203,15 @@ function update_variables_to_config($config_name, array $variable_map) {
   }
   $default_data = $file->read($config_name);
 
+  // Apply the default values.
+  $config->setData($default_data);
+
   // Merge any possibly existing original data into default values.
   // Only relevant when being called repetitively on the same config object.
   if (!empty($original_data)) {
-    $data = drupal_array_merge_deep($default_data, $original_data);
-  }
-  else {
-    $data = $default_data;
+    $config->merge($original_data);
   }
 
-  // Apply the default values.
-  $config->setData($data);
-
   // Fetch existing variables.
   $variables = db_query('SELECT name, value FROM {variable} WHERE name IN (:variables)', array(':variables' => array_keys($variable_map)))->fetchAllKeyed();
 
diff --git a/core/lib/Drupal/Component/Utility/NestedArray.php b/core/lib/Drupal/Component/Utility/NestedArray.php
index 6617c68..e6ab1ee 100644
--- a/core/lib/Drupal/Component/Utility/NestedArray.php
+++ b/core/lib/Drupal/Component/Utility/NestedArray.php
@@ -315,19 +315,19 @@ public static function mergeDeep() {
    *
    * @see NestedArray::mergeDeep()
    */
-  public static function mergeDeepArray(array $arrays) {
+  public static function mergeDeepArray(array $arrays, $preserve_integer_keys = FALSE) {
     $result = array();
     foreach ($arrays as $array) {
       foreach ($array as $key => $value) {
-        // Renumber integer keys as array_merge_recursive() does. Note that PHP
-        // automatically converts array keys that are integer strings (e.g., '1')
-        // to integers.
-        if (is_integer($key)) {
+        // Renumber integer keys as array_merge_recursive() does unless
+        // $preserve_integer_keys is set to TRUE. Note that PHP automatically
+        // converts array keys that are integer strings (e.g., '1') to integers.
+        if (is_integer($key) && !$preserve_integer_keys) {
           $result[] = $value;
         }
         // Recurse when both values are arrays.
         elseif (isset($result[$key]) && is_array($result[$key]) && is_array($value)) {
-          $result[$key] = self::mergeDeepArray(array($result[$key], $value));
+          $result[$key] = self::mergeDeepArray(array($result[$key], $value), $preserve_integer_keys);
         }
         // Otherwise, use the latter value, overriding any previous value.
         else {
diff --git a/core/lib/Drupal/Core/Config/Config.php b/core/lib/Drupal/Core/Config/Config.php
index 3fe9563..c63b2a0 100644
--- a/core/lib/Drupal/Core/Config/Config.php
+++ b/core/lib/Drupal/Core/Config/Config.php
@@ -401,4 +401,19 @@ public function getStorage() {
   protected function notify($config_event_name) {
     $this->eventDispatcher->dispatch('config.' . $config_event_name, new ConfigEvent($this));
   }
+
+  /*
+   * Merges data into a configuration object.
+   *
+   * @param array $data_to_merge
+   *   An array containing data to merge.
+   *
+   * @return Drupal\Core\Config\Config
+   *   The configuration object.
+   */
+  public function merge(array $data_to_merge) {
+    // Preserve integer keys so that config keys are not changed.
+    $this->data = NestedArray::mergeDeepArray(array($this->data, $data_to_merge), TRUE);
+    return $this;
+  }
 }
diff --git a/core/modules/system/config/system.site.yml b/core/modules/system/config/system.site.yml
index 010dedf..ee9b6a7 100644
--- a/core/modules/system/config/system.site.yml
+++ b/core/modules/system/config/system.site.yml
@@ -5,3 +5,4 @@ page:
   403: ''
   404: ''
   front: user
+admin_compact_mode: '0'
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index e938128..ac1aa94 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -2215,6 +2215,17 @@ function system_update_8032() {
 }
 
 /**
+ * Move the admin_compact_mode setting from variable to config.
+ *
+ * @ingroup config_upgrade
+ */
+function system_update_8033() {
+  update_variables_to_config('system.site', array(
+    'admin_compact_mode' => 'admin_compact_mode',
+  ));
+}
+
+/**
  * @} End of "defgroup updates-7.x-to-8.x".
  * The next series of updates should start at 9000.
  */
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 7f9f33a..2b06273 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -3363,7 +3363,7 @@ function confirm_form($form, $question, $path, $description = NULL, $yes = NULL,
 function system_admin_compact_mode() {
   // PHP converts dots into underscores in cookie names to avoid problems with
   // its parser, so we use a converted cookie name.
-  return isset($_COOKIE['Drupal_visitor_admin_compact_mode']) ? $_COOKIE['Drupal_visitor_admin_compact_mode'] : variable_get('admin_compact_mode', FALSE);
+  return isset($_COOKIE['Drupal_visitor_admin_compact_mode']) ? $_COOKIE['Drupal_visitor_admin_compact_mode'] : config('system.site')->get('admin_compact_mode');
 }
 
 /**