diff --git a/CRM/Upgrade/Incremental/php/SixZero.php b/CRM/Upgrade/Incremental/php/SixZero.php
index d5436d763254..38ae9ece6813 100644
--- a/CRM/Upgrade/Incremental/php/SixZero.php
+++ b/CRM/Upgrade/Incremental/php/SixZero.php
@@ -44,6 +44,8 @@ public function upgrade_6_0_alpha1($rev): void {
     );
     $this->addTask('Set a default activity priority', 'addActivityPriorityDefault');
     $this->addSimpleExtensionTask(ts('enable dedupe backward compatibility'), ['legacydedupefinder']);
+    $this->addExtensionTask('Enable Riverlea extension', ['riverlea']);
+    $this->addTask('Freeze theme choice to Greenwich', 'freezeDefaultThemeToGreenwich');
   }
 
   public static function migrateFromEmailAddressValues($rev): bool {
@@ -112,4 +114,25 @@ public static function addActivityPriorityDefault() {
     return TRUE;
   }
 
+  /**
+   * On 6.0 the default theme changes to Riverlea.
+   *
+   * For sites upgrading that have not made an explicit theme choice and have previously got
+   * Greenwich by default, change this to an explicit choice of Greenwich.
+   *
+   * @return bool
+   */
+  public static function freezeDefaultThemeToGreenwich() {
+    $themeSettings = ['theme_backend', 'theme_frontend'];
+
+    foreach ($themeSettings as $key) {
+      $value = \Civi::settings()->get($key);
+      if ($value === 'default') {
+        \Civi::settings()->set($key, 'greenwich');
+      }
+    }
+
+    return TRUE;
+  }
+
 }
diff --git a/settings/Core.setting.php b/settings/Core.setting.php
index d604f723877d..a938b23ce5ea 100644
--- a/settings/Core.setting.php
+++ b/settings/Core.setting.php
@@ -1184,7 +1184,7 @@
     'pseudoconstant' => array(
       'callback' => 'call://themes/getAvailable',
     ),
-    'default' => 'default',
+    'default' => 'minetta',
     'add' => '5.16',
     'title' => ts('Frontend Theme'),
     'is_domain' => 1,
@@ -1209,7 +1209,7 @@
     'pseudoconstant' => array(
       'callback' => 'call://themes/getAvailable',
     ),
-    'default' => 'default',
+    'default' => 'minetta',
     'add' => '5.16',
     'title' => ts('Backend Theme'),
     'is_domain' => 1,
diff --git a/setup/plugins/init/DefaultExtensions.civi-setup.php b/setup/plugins/init/DefaultExtensions.civi-setup.php
index fe7d6518f8f2..54505c5ae0c8 100644
--- a/setup/plugins/init/DefaultExtensions.civi-setup.php
+++ b/setup/plugins/init/DefaultExtensions.civi-setup.php
@@ -22,5 +22,6 @@
     $e->getModel()->extensions[] = 'authx';
     $e->getModel()->extensions[] = 'civiimport';
     $e->getModel()->extensions[] = 'message_admin';
+    $e->getModel()->extensions[] = 'riverlea';
 
   });