diff -u b/core/includes/mail.inc b/core/includes/mail.inc
--- b/core/includes/mail.inc
+++ b/core/includes/mail.inc
@@ -119,7 +119,10 @@
  */
 function drupal_mail($module, $key, $to, $language, $params = array(), $from = NULL, $send = TRUE) {
   $site_mail = config('system.site')->get('mail');
-  $default_from = (!empty($site_mail) ? $site_mail : ini_get('sendmail_from'));
+  if (empty($site_mail)) {
+    $site_mail = ini_get('sendmail_from');
+  }
+  $default_from = $site_mail;
 
   // Bundle up the variables into a structured array for altering.
   $message = array(
diff -u b/core/includes/path.inc b/core/includes/path.inc
--- b/core/includes/path.inc
+++ b/core/includes/path.inc
@@ -298,7 +298,13 @@
   $is_front_page = &$drupal_static_fast['is_front_page'];
 
   if (!isset($is_front_page)) {
-    $is_front_page = (current_path() == config('system.site')->get('page.front'));
+    // If we are in the install mode, then always return FALSE.
+    if (defined('MAINTENANCE_MODE') && MAINTENANCE_MODE == 'install') {
+      $is_front_page = FALSE;
+    }
+    else {
+      $is_front_page = (current_path() == config('system.site')->get('page.front'));
+    }
   }
 
   return $is_front_page;
diff -u b/core/includes/theme.inc b/core/includes/theme.inc
--- b/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -2747,18 +2747,26 @@
     $variables['layout'] = ($variables['layout'] == 'first') ? 'both' : 'second';
   }
 
-  $site_config = config('system.site');
+  $site_name = '';
+  $site_slogan = '';
+  // If we are not in the install mode, then load the site_name and site_slogan
+  // from config.
+  if (!(defined('MAINTENANCE_MODE') && MAINTENANCE_MODE == 'install')) {
+    $site_config = config('system.site');
+    $site_name = $site_config->get('name');
+    $site_slogan = $site_config->get('slogan');
+  }
   // Construct page title
   if (drupal_get_title()) {
     $head_title = array(
       'title' => strip_tags(drupal_get_title()),
-      'name' => $site_config->get('name'),
+      'name' => $site_name,
     );
   }
   else {
-    $head_title = array('name' => $site_config->get('name'));
-    if ($site_config->get('slogan')) {
-      $head_title['slogan'] = $site_config->get('slogan');
+    $head_title = array('name' => $site_name);
+    if ($site_slogan) {
+      $head_title['slogan'] = $site_slogan;
     }
   }
 
@@ -2775,8 +2783,8 @@
   $variables['messages']          = $variables['show_messages'] ? theme('status_messages') : '';
   $variables['main_menu']         = array();
   $variables['secondary_menu']    = array();
-  $variables['site_name']         = (theme_get_setting('toggle_name') ? $site_config->get('name') : '');
-  $variables['site_slogan']       = (theme_get_setting('toggle_slogan') ? $site_config->get('slogan') : '');
+  $variables['site_name']         = (theme_get_setting('toggle_name') ? $site_name : '');
+  $variables['site_slogan']       = (theme_get_setting('toggle_slogan') ? $site_slogan : '');
   $variables['tabs']              = '';
   $variables['title']             = drupal_get_title();
 
diff -u b/core/modules/node/node.module b/core/modules/node/node.module
--- b/core/modules/node/node.module
+++ b/core/modules/node/node.module
@@ -2541,7 +2541,7 @@
     ->orderBy('sticky', 'DESC')
     ->orderBy('created', 'DESC')
     ->extend('Drupal\Core\Database\Query\PagerSelectExtender')
-    ->limit($site_config->get('default_nodes_main'))
+    ->limit($site_config->get('max_posts'))
     ->addTag('node_access');
 
   $nids = $select->execute()->fetchCol();
diff -u b/core/modules/system/config/system.site.yml b/core/modules/system/config/system.site.yml
--- b/core/modules/system/config/system.site.yml
+++ b/core/modules/system/config/system.site.yml
@@ -4,5 +4,5 @@
-default_nodes_main: '10'
+max_posts: '10'
 page:
     front: user
-    403: ''
     404: ''
+    403: ''
diff -u b/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
--- b/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -1506,9 +1506,9 @@
     '#description' => t('Optionally, specify a relative URL to display as the front page.  Leave blank to display the default content feed.'),
     '#field_prefix' => url(NULL, array('absolute' => TRUE)),
   );
-  $form['front_page']['default_nodes_main'] = array(
+  $form['front_page']['max_posts'] = array(
     '#type' => 'select', '#title' => t('Number of posts on front page'),
-    '#default_value' => $site_config->get('default_nodes_main'),
+    '#default_value' => $site_config->get('max_posts'),
     '#options' => drupal_map_assoc(array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30)),
     '#description' => t('The maximum number of posts displayed on overview pages such as the front page.'),
     '#access' => ($site_config->get('page.front') == 'node'),
@@ -1583,7 +1583,7 @@
     ->set('name', $form_state['values']['site_name'])
     ->set('mail', $form_state['values']['site_mail'])
     ->set('slogan', $form_state['values']['site_slogan'])
-    ->set('default_nodes_main', $form_state['values']['default_nodes_main'])
+    ->set('max_posts', $form_state['values']['max_posts'])
     ->set('page.front', $form_state['values']['site_frontpage'])
     ->set('page.403', $form_state['values']['site_403'])
     ->set('page.404', $form_state['values']['site_404'])
diff -u b/core/modules/system/system.install b/core/modules/system/system.install
--- b/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -1934,7 +1934,7 @@
     'site_name' => 'name',
     'site_mail' => 'mail',
     'site_slogan' => 'slogan',
-    'default_nodes_main' => 'default_nodes_main',
+    'default_nodes_main' => 'max_posts',
     'site_frontpage' => 'page.front',
     'site_403' => 'page.403',
     'site_404' => 'page.404',
diff -u b/core/modules/taxonomy/taxonomy.pages.inc b/core/modules/taxonomy/taxonomy.pages.inc
--- b/core/modules/taxonomy/taxonomy.pages.inc
+++ b/core/modules/taxonomy/taxonomy.pages.inc
@@ -42,7 +42,7 @@
     'term' => taxonomy_term_view($term, 'full'),
   );
 
-  if ($nids = taxonomy_select_nodes($term->tid, TRUE, config('system.site')->get('default_nodes_main'))) {
+  if ($nids = taxonomy_select_nodes($term->tid, TRUE, config('system.site')->get('max_posts'))) {
     $nodes = node_load_multiple($nids);
     $build += node_view_multiple($nodes);
     $build['pager'] = array(
only in patch2:
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -77,7 +77,8 @@ class ThemeTest extends WebTestBase {
     $original_path = _current_path();
     // Set the current path to node because theme_get_suggestions() will query
     // it to see if we are on the front page.
-    variable_set('site_frontpage', 'node');
+    config('system.site')->set('page.front', 'node')->save();
+    drupal_static_reset('drupal_is_front_page');
     _current_path('node');
     $suggestions = theme_get_suggestions(array('node'), 'page');
     // Set it back to not annoy the batch runner.
only in patch2:
unchanged:
--- a/core/modules/system/lib/Drupal/system/Tests/Upgrade/UpgradePathTestBase.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Upgrade/UpgradePathTestBase.php
@@ -134,10 +134,6 @@ abstract class UpgradePathTestBase extends WebTestBase {
     // Generate and set a D8-compatible session cookie.
     $this->prepareD8Session();
 
-    // Restore necessary variables.
-    // @todo Convert into config('system.site')->set('mail')?
-    $this->variable_set('site_mail', 'simpletest@example.com');
-
     drupal_set_time_limit($this->timeLimit);
     $this->setup = TRUE;
   }