diff --git a/admin/include/functions_upgrade.php b/admin/include/functions_upgrade.php
index 8581527cc..e36960dcd 100644
--- a/admin/include/functions_upgrade.php
+++ b/admin/include/functions_upgrade.php
@@ -100,9 +100,7 @@ function deactivate_non_standard_themes()
   global $page, $conf;
 
   $standard_themes = array(
-    'clear',
-    'Sylvia',
-    'dark',
+    'modus',
     'elegant',
     'smartpocket',
     );
@@ -146,9 +144,26 @@ SELECT theme
     // if the default theme has just been deactivated, let's set another core theme as default
     if (in_array($default_theme, $theme_ids))
     {
+      // make sure default Piwigo theme is active
+      $query = '
+SELECT
+    COUNT(*)
+  FROM '.PREFIX_TABLE.'themes
+  WHERE id = \''.PHPWG_DEFAULT_TEMPLATE.'\'
+;';
+      list($counter) = pwg_db_fetch_row(pwg_query($query));
+      if ($counter < 1)
+      {
+        // we need to activate theme first
+        include_once(PHPWG_ROOT_PATH.'admin/include/themes.class.php');
+        $themes = new themes();
+        $themes->perform_action('activate', PHPWG_DEFAULT_TEMPLATE);
+      }
+
+      // then associate it to default user
       $query = '
 UPDATE '.PREFIX_TABLE.'user_infos
-  SET theme = \'elegant\'
+  SET theme = \''.PHPWG_DEFAULT_TEMPLATE.'\'
   WHERE user_id = '.$conf['default_user_id'].'
 ;';
       pwg_query($query);
diff --git a/admin/include/updates.class.php b/admin/include/updates.class.php
index 82d52e1bb..294917375 100644
--- a/admin/include/updates.class.php
+++ b/admin/include/updates.class.php
@@ -1,10 +1,10 @@
-<?php
-// +-----------------------------------------------------------------------+
-// | This file is part of Piwigo.                                          |
-// |                                                                       |
-// | For copyright and license information, please view the COPYING.txt    |
-// | file that was distributed with this source code.                      |
-// +-----------------------------------------------------------------------+
+<?php
+// +-----------------------------------------------------------------------+
+// | This file is part of Piwigo.                                          |
+// |                                                                       |
+// | For copyright and license information, please view the COPYING.txt    |
+// | file that was distributed with this source code.                      |
+// +-----------------------------------------------------------------------+
 
 if (!defined('PHPWG_ROOT_PATH')) die('Hacking attempt!');
 
@@ -29,7 +29,7 @@ class updates
     {
       $this->types = array($page);
     }
-    $this->default_themes = array('clear', 'dark', 'Sylvia', 'elegant', 'smartpocket');
+    $this->default_themes = array('modus', 'elegant', 'smartpocket');
     $this->default_plugins = array('AdminTools', 'TakeATour', 'language_switch', 'LocalFilesEditor');
 
     foreach ($this->types as $type)
diff --git a/include/constants.php b/include/constants.php
index 844356c10..8c43a4521 100644
--- a/include/constants.php
+++ b/include/constants.php
@@ -9,9 +9,10 @@
 // Default settings
 define('PHPWG_VERSION', '2.10.0');
 define('PHPWG_DEFAULT_LANGUAGE', 'en_UK');
-// this constant is actually never used, the true default theme is the theme or user "guest",
-// which is initialized with column user_infos.theme default value
-// (see file install/piwigo_structure-mysql.sql)
+
+// this constant is only used in the upgrade process, the true default theme
+// is the theme of user "guest", which is initialized with column user_infos.theme
+// default value (see file install/piwigo_structure-mysql.sql)
 define('PHPWG_DEFAULT_TEMPLATE', 'modus');
 
 define('PHPWG_THEMES_PATH', $conf['themes_dir'].'/');