Although this is post is quite old, I'm having the same problem here. See below for the changes that I've hacked into Security.php (mostly it's commenting out the functionality for the tabs). Problem now is that I have to create CustomLoginForm.ss for all my other websites too, and that's not the idea here.
O, and my actual requirement: how can I load Security_login in a 100% custom template WITHOUT all kinds of JS and CSS stuff being put in (which is currently done by Security.php).
PS. I have tried <% control LoginForm %> in Security_login, but than the error messages ("wrong email/pass") are not shown :(
// // only display tabs when more than one authenticator is provided
// // to save bandwidth and reduce the amount of custom styling needed
// if(count($forms) > 1) {
// Requirements::javascript(SAPPHIRE_DIR . '/javascript/loader.js');
// Requirements::javascript(SAPPHIRE_DIR . "/thirdparty/prototype/prototype.js");
// Requirements::javascript(SAPPHIRE_DIR . "/thirdparty/behaviour/behaviour.js");
// Requirements::javascript(SAPPHIRE_DIR . "/javascript/prototype_improvements.js");
// Requirements::javascript(THIRDPARTY_DIR . "/scriptaculous/effects.js");
// Requirements::css(SAPPHIRE_DIR . "/css/Form.css");
//
// // Needed because the <base href=".."> in the template makes problems
// // with the tabstrip library otherwise
// $link_base = Director::absoluteURL($this->Link("login"));
//
// Requirements::javascript(THIRDPARTY_DIR . "/jquery/jquery.js");
// Requirements::javascript(SAPPHIRE_DIR . "/javascript/jquery_improvements.js");
// Requirements::javascript(THIRDPARTY_DIR . "/tabstrip/tabstrip.js");
// Requirements::css(THIRDPARTY_DIR . "/tabstrip/tabstrip.css");
//
// $content = '<div id="Form_EditForm">';
// $content .= '<ul class="tabstrip">';
// $content_forms = '';
//
// foreach($forms as $form) {
// $content .= "<li><a href=\"$link_base#{$form->FormName()}_tab\">{$form->getAuthenticator()->get_name()}</a></li>\n";
// $content_forms .= '<div class="tab" id="' . $form->FormName() . '_tab">' . $form->forTemplate() . "</div>\n";
// }
//
// $content .= "</ul>\n" . $content_forms . "\n</div>\n";
// } else {
// $content .= $forms[0]->forTemplate();
// }
$content .= $forms[0]->renderWith('CustomLoginForm');