If you would like to change your recaptcha theme to match your site.

In file:

components/com_communityanswers/templates/default/new_question.php

After line 69, before the code:

<?php if($user->guest && $answersConfig[ENABLE_CAPTCHA]):?>

Paste the following:

<script type="text/javascript">

 var RecaptchaOptions = {

    theme : '<theme name here>'

 };

 </script>

Replace <theme name here> with one of these themes:

http://wiki.recaptcha.net/index.php/Theme

Enjoy!