yii2-app-basic/config/assets.php

<?php

return array(
	'app' => array(
		'basePath' => '@wwwroot',
		'baseUrl' => '@www',
		'css' => array(
			'css/site.css',
		),
		'js' => array(

		),
		'depends' => array(
			'yii',
			'yii/bootstrap/responsive',
		),
	),
);