In PaddyGreen we will have to check this out but for your CSS files you should have this in your Page.php (in mysite/code)
function init() {
parent::init();
Requirements::themedCSS('layout');
Requirements::themedCSS('typography');
Requirements::themedCSS('form');
Requirements::themedCSS('myuberfancycssfile');
}
Requirements loads CSS files based on what theme you have set. So in your _config.php you will have SSViewer::set_theme('PaddyGreen') and having that init() in your php will automagiclly look for the css files in PaddyGreen/css. Or you can edit the main Page.ss (If using PaddyGreen it will be themes/PaddyGreen/templates/Page.ss) and add the CSS files as per any standard CSS Linking method :D. Why we have requirements for the css is something I dont even know! I personally prefer the second method.. I just like to have full control over them in my templates.