php - AppAsset $css property

one text

I've recently started learning Yii2 Framework, and I face the following issue:

I'm trying to connect my css & js files via AppAsset class, here it is:

class AppAsset extends AssetBundle
{
    public $basePath = '@webroot';
    public $baseUrl = '@web';
    public $css = [
//        'css/bootstrap.min.css',
        'css/font-awesome.min.css',
        'css/prettyPhoto.css',
        'css/animate.css',
        'css/main.css',
        'css/responsive.css',
    ];
    public $js = [
//        'js/jquery.js',
//        'js/bootstrap.min.js',
        'js/jquery.scrollUp.min.js',
        'js/price-range.js',
        'js/jquery.prettyPhoto.js',
        'js/main.js',
    ];
    public $depends = [
        'yii\web\YiiAsset',
        'yii\bootstrap4\BootstrapPluginAsset',
    ];
}

All the assets are put into the web folder and grouped by the subfolders (like js, css etc), but for some reason only js files are connected, while css is completely ignored (it's not even displayed in devtools sources), which seems weird to me. I assume it's not a path issue since js works properly, folder permissions are also fine. What could be the reason of such behaviour?