yii2-bootstrap/docs/guide/topics-less.md

Using the .less files of Bootstrap directly

If you want to include the Bootstrap css directly in your less files you may need to disable the original bootstrap css files to be loaded. You can do this by setting the css property of the yii\bootstrap\BootstrapAsset to be empty. For this you need to configure the assetManager application component as follows:

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