If you want to include the Bootstrap css directly in your sass files
you may need to disable the bootstrap css files loaded by this extension.
You can do this by setting the css property of BootstrapAsset to be empty.
For this, you need to configure the
assetManager application component as follows:
'assetManager' => [ 'bundles' => [ 'yii\bootstrap4\BootstrapAsset' => [ 'css' => , ] ] ]