add css file located in different folder to an Asset Bundle

I have a blockui asset which load blockui js library from bower package. I want to apply a custom css file that should be included when I use BlockUI asset.

However, because blockui js lib is located in vendor/bower/blockui and my custom css is at frontend/web/css, I can not add custom css easily to blockui asset bundle and i am stucked

This is my asset bundle code

namespace frontend\assets;

use yii\web\AssetBundle;


 * @author Qiang Xue <>

 * @since 2.0


class BlockUIAsset extends AssetBundle


    public $sourcePath = '@bower';

    public $baseUrl = '@web';

    public $js = [



    #this does not work because it look for vendor/bower/css/custom.css which does not exist

    #my custom.css is located at frontend/web/css/

    public $css = [



    public $depends = [