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
@@ -2104,8 +2104,8 @@
   // Add the core theme to the variable to configuration conversion map and
   // install the default configuration.
   foreach ($core_themes as $theme) {
-    $variable = 'theme_' . $theme->name . '_settings';
-    $config_name = $theme->name . '.settings';
+    $variable = 'theme_' . $theme . '_settings';
+    $config_name = $theme . '.settings';
     $theme_settings_to_config_map[$variable] = $config_name;
     update_7_to_8_install_default_config('theme', $config_name);
   }