the post is old but if you have to be helpful set the hook of the module with this line ! $ this-> registerHook ('header') example:
public function install()
{
if (!parent::install() || !$this->registerHook('home') || !$this->registerHook('header'))
return false;
return true;
}
then add this
public function hookheader($param)
{
global $smarty;
Tools::addCSS($this->_path.'mymodules.css', 'all');
}
with this system, the module goes to get the css in the template directoy example :
themes/mytemplate/css/modules/mymodules/mymodules.css
If the module has pictures put the image folder in the same directory example:
themes/mytemplate/css/modules/mymodules/img/
I hope to be helpful goodbye
greetings from italy.