theme.php

public function action_admin_header($theme)
{
  if ( $theme->page == 'themes' ) {
    $js = <<<JS
      $(document).ready(function() {
        $('#twitter input:checkbox').bind('change', function() {
          var disable_twitter_username = $('#twitter input:checked').length == 0;
          $('#twitter_username input').attr('disabled', disable_twitter_username);
        });

        $('#twitter input:checkbox').change();
      });
JS;
    Stack::add('admin_header_javascript', $js, 'theme_options');
  }
}