/* Custom 404 Hooks */
    function custom_thesis_404_title() {
    ?>
    YOUR 404 PAGE HEADING HERE
    <?
    }
    remove_action('thesis_hook_404_title', 'thesis_404_title');
    add_action('thesis_hook_404_title', 'custom_thesis_404_title');
    function custom_thesis_404_content() {
    ?>
<style type="text/css">
<!-- #sidebars { display:none; } -->
</style>
    <p>WHATEVER YOU WANT YOUR 404 PAGE TO SAY HERE</p>
    <?
    }
    remove_action('thesis_hook_404_content', 'thesis_404_content');
    add_action('thesis_hook_404_content', 'custom_thesis_404_content');