And you can use %stylesheets% (assetic feature) tag:
{% stylesheets
"@MainBundle/Resources/public/colorbox/colorbox.css"
"%kerner.root_dir%/Resources/css/main.css"
%}
<link type="text/css" rel="stylesheet" media="all" href="{{ asset_url }}" />
{% endstylesheets %}
You can write path to css as parameter (%parameter_name%).
More about this variant: http://symfony.com/doc/current/cookbook/assetic/asset_management.html