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 BootstrapAsset to be empty.
For this you need to configure the
assetManager application component as follows:
'assetManager' => [ 'bundles' => [ 'yii\bootstrap\BootstrapAsset' => [ 'css' => , ] ] ]