There's a small bug, which makes using custom templates impossible. In com_communitypolls/controllers/polls.pp, line 412:

$tpl = !empty($poll->params['template']) ? JFile::makeSafe($params['template']) : 'default';

Should be this instead:

$tpl = !empty($params['template']) ? JFile::makeSafe($params['template']) : 'default';

Please, fix this.