4 different ways :
In actual version, I am not sure if all are working. (1°, 4° for sure )
1. static file
dom/assets/ASSET_NAME/css/MYCSS.css (your css file)
in your class :
protected $assetName = 'ASSET_NAME'; (can be different than the name of the class)
protected $attachCss = array(
'MYCSS.css'
);
2. static file with check
In buildCss() : (better, but can also be in constructor)
$this->attachCss[] = 'MYCSS.css';
3. Full name CSS (not necessary in assets)
$this->addStyleSheet('FULL_URL.css');
4. Only style
$this->addStyle('font-size', '1em');
