yii2-bootstrap5/docs/guide/topics-sass.md

Using the .sass files of Bootstrap directly

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 yii\bootstrap4\BootstrapAsset to be empty. For this, you need to configure the assetManager application component as follows:

    'assetManager' => [
        'bundles' => [
            'yii\bootstrap4\BootstrapAsset' => [
                'css' => [],
            ]
        ]
    ]