Including Frontend CSS file into Backend in Yii2

I’m trying to include a css file lacated in frontend/web/css to backend main layout. It is working for backend but produce error for frontend. frontend web url is staging.example.com and backend web url is backend.example.com.

<?php

namespace frontend\assets;

use yii\web\AssetBundle;

class IconAsset extends AssetBundle

{

public &#036;sourcePath = '@frontend/web';


public &#036;css = [


    'css/icon.css',


];

}

?>

Code inside frontend layout main.php


use frontend\assets\IconAsset;

IconAsset::register($this);

Code inside backend layout main.php


use frontend\assets\IconAsset;

IconAsset::register($this);