Open Thesis > Custom File Editor and select custom-functions.php and add the following code:
/* Custom 404 Title*/
function custom_404_title() {
?>
<strong>Article Not Found</strong>
<?php
}
remove_action('thesis_hook_404_title', 'thesis_404_title');
add_action('thesis_hook_404_title', 'custom_404_title');
/* Custom 404 Content*/
function custom_404_content() {
?>
<p><strong>Article Not Found!</strong><br />The article you were looking for was not found, try again later!</p>
<?php
}
remove_action('thesis_hook_404_content', 'thesis_404_content');
add_action('thesis_hook_404_content', 'custom_404_content');