Bug 1501078: When sitethemeprefs is false / no theme select box

Need to check that $values['theme'] variable exists

behatnotneeded

Change-Id: Iba75b8306abdb01d876f4f402fc0a01a9bd2d49b
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
(cherry picked from commit c31c81c7)

htdocs/account/index.php

@@ -239,7 +239,7 @@ function accountprefs_submit(Pieform $form, $values) {
         }
     }
 
-    if ($values['theme'] == 'sitedefault') {
+    if (isset($values['theme']) && $values['theme'] == 'sitedefault') {
         $USER->set_account_preference('theme', '');
     }