app\modules\backend\assets\AdminAsset::init PHP Method

    public function init()
    {
        parent::init();
        // we recompile the less files from 'yii\bootstrap\BootstrapAsset' and include the css in app.css
        // therefore we set bundle to false
        \Yii::$app->getAssetManager()->bundles['yii\\bootstrap\\BootstrapAsset'] = false;
        // /!\ CSS/LESS development only setting /!\
        // Touch the asset folder with the highest mtime of all contained files
        // This will create a new folder in web/assets for every change and request
        // made to the app assets.
        if (getenv('APP_ASSET_FORCE_PUBLISH')) {
            $files = FileHelper::findFiles(\Yii::getAlias($this->sourcePath));
            $mtimes = [];
            foreach ($files as $file) {
                $mtimes[] = filemtime($file);
            }
            touch(\Yii::getAlias($this->sourcePath), max($mtimes));
        }
    }