I'm really wondering why you would want to do so if the CSS files are served on the same domain as your website, but this will probably work;
<?php echo $this->Html->css($this->Html->url('/css/main.css', true));?>
HtmlHelper::url() (inherited from Helper::url() takes two arguments. The first is the path as a string or array (to make use of cakephp routing), if a Boolean 'true' is passed as its second argument, a 'full' URL is generated.
I'm not at my computer at the moment, so haven't been able to test it, but I think it should work.
update
I had a quick look at the source: https://github.com/cakephp/cakephp/blob/master/lib/Cake/View/Helper/HtmlHelper.php#L431
and HtmlHelper::css() uses Helper::assetUrl() internally;
https://github.com/cakephp/cakephp/blob/master/lib/Cake/View/Helper.php#L305
So this may work as well and if it works, it's a cleaner solution:
<?php echo $this->Html->css('main', null, array('fullBase' => true));?>