diff --git a/lib/outputrenderers.php b/lib/outputrenderers.php
index 4e18268cba4..ec3aa463e65 100644
--- a/lib/outputrenderers.php
+++ b/lib/outputrenderers.php
@@ -4922,6 +4922,47 @@ class core_renderer_maintenance extends core_renderer {
 
     }
 
+    /**
+     * Overridden confirm message for upgrades.
+     *
+     * @param string $message The question to ask the user
+     * @param single_button|moodle_url|string $continue The single_button component representing the Continue answer.
+     * @param single_button|moodle_url|string $cancel The single_button component representing the Cancel answer.
+     * @return string HTML fragment
+     */
+    public function confirm($message, $continue, $cancel) {
+        // We need plain styling of confirm boxes on upgrade because we don't know which stylesheet we have (it could be
+        // from any previous version of Moodle).
+        if ($continue instanceof single_button) {
+            $continue->primary = true;
+        } else if (is_string($continue)) {
+            $continue = new single_button(new moodle_url($continue), get_string('continue'), 'post', true);
+        } else if ($continue instanceof moodle_url) {
+            $continue = new single_button($continue, get_string('continue'), 'post', true);
+        } else {
+            throw new coding_exception('The continue param to $OUTPUT->confirm() must be either a URL' .
+                                       ' (string/moodle_url) or a single_button instance.');
+        }
+
+        if ($cancel instanceof single_button) {
+            $output = '';
+        } else if (is_string($cancel)) {
+            $cancel = new single_button(new moodle_url($cancel), get_string('cancel'), 'get');
+        } else if ($cancel instanceof moodle_url) {
+            $cancel = new single_button($cancel, get_string('cancel'), 'get');
+        } else {
+            throw new coding_exception('The cancel param to $OUTPUT->confirm() must be either a URL' .
+                                       ' (string/moodle_url) or a single_button instance.');
+        }
+
+        $output = $this->box_start('generalbox', 'notice');
+        $output .= html_writer::tag('h4', get_string('confirm'));
+        $output .= html_writer::tag('p', $message);
+        $output .= html_writer::tag('div', $this->render($continue) . $this->render($cancel), array('class' => 'buttons'));
+        $output .= $this->box_end();
+        return $output;
+    }
+
     /**
      * Does nothing. The maintenance renderer does not support JS.
      *
diff --git a/theme/bootstrapbase/less/moodle/admin.less b/theme/bootstrapbase/less/moodle/admin.less
index 3fe8f59329b..0b31b18eb78 100644
--- a/theme/bootstrapbase/less/moodle/admin.less
+++ b/theme/bootstrapbase/less/moodle/admin.less
@@ -204,7 +204,21 @@ img.iconsmall {
 .modal.modal-in-page {
     position: static;
     z-index: 0;
-    margin: 0;
+    margin: 0 auto 0 auto;
+}
+
+// We need to keep this rule because confirm messages during upgrade use it. We can never change it because we don't
+// serve new stylesheets until after upgrade.
+#page-admin-index #notice {
+    width: 60%;
+    min-width: 220px;
+    margin: auto;
+
+    .buttons {
+        padding-left: 0;
+        padding-right: 0;
+        text-align: center;
+    }
 }
 
 #page-admin-index .releasenoteslink,
diff --git a/theme/bootstrapbase/style/moodle.css b/theme/bootstrapbase/style/moodle.css
index c9b970d1eae..b483c98e3ca 100644
--- a/theme/bootstrapbase/style/moodle.css
+++ b/theme/bootstrapbase/style/moodle.css
@@ -2789,7 +2789,17 @@ img.iconsmall {
 .modal.modal-in-page {
   position: static;
   z-index: 0;
-  margin: 0;
+  margin: 0 auto 0 auto;
+}
+#page-admin-index #notice {
+  width: 60%;
+  min-width: 220px;
+  margin: auto;
+}
+#page-admin-index #notice .buttons {
+  padding-left: 0;
+  padding-right: 0;
+  text-align: center;
 }
 #page-admin-index .releasenoteslink,
 #page-admin-index .adminwarning,