You have to create restore.php file ( the name is an example it can be any name ) on your computer and add the following content:
 

<?php

require_once("mainfile.php");
$theme= "DeepBlue";
$sql = "UPDATE ".$prefix."_config SET Default_Theme = '".$theme."'";
$result = $db->sql_query($sql);
echo "Thanks for using this fix! Your theme has been reset to ".$theme."";

?>
 

Once the file is created you should upload it under the www root folder on your account and visit the following link (if the file is not located under sub-folder of your www root directory ):

http://yourdomain.com/restore.php

Once the above URL is visited the default theme of your php-Nuke will be restored.