field_view_value returns null for boolean field
Set 

$display['type'] = 'list_key';


in your theme function to get not null key of field. Or create own formatter, for ex.

function mymodule_field_formatter_info() {
  return array(
      'my_formatter' =&gt; array(
          'label' =&gt; t('My formatter'),
          'field types' =&gt; array('list_boolean'),
      ),
  );
}


function mymodule_field_formatter_view($entity_type, $entity, $field, $instance, $langcode, $items, $display) {
  $element = array();
  $settings = $display['settings'];

  switch ($display['type']) {
    case 'my_formatter':
      foreach($items as $delta =&gt; $item) {
        $value = $item['value'] == 1 ? '&lt;span class=""&gt;ON TEXT&lt;/span&gt;' : '&lt;span class=""&gt;OFF TEXT&lt;/span&gt;';
        $element[$delta] = array(
            '#markup' =&gt; $value,
        );
      }
      break;
  }

  return $element;
}


and after that set 

$display['type'] = 'my_formatter';


in Your theme function