Okay here is the hook. Add below code to your theme file or plugin layer.
// To replace qa-template-custom to qa-template-my-page
function body_tags()
{
$class = (($this->template == 'custom') ? 'qa-template-' . qa_request() : 'qa-template-' . qa_html($this->template));
if (isset($this->content['categoryids']))
foreach ($this->content['categoryids'] as $categoryid)
$class.=' qa-category-' . qa_html($categoryid);
$this->output('class="' . $class . ' qa-body-js-off"');
}