DMSGuestbook changing theme

How would I change the theme for reCAPTCHA?
On the reCAPTCHA customization web site it shows to do this but I don’t know where to put it.

<script type="text/javascript">
 var RecaptchaOptions = {
    theme : 'theme_name'
 };
 </script>

http://wordpress.org/extend/plugins/dmsguestbook/