отключить bootsctrap http://bigcode.ru/disable_bootstrap_for_yii/
'assetManager' => [
'bundles' => [
'yii\bootstrap\BootstrapAsset' => [
'css' => [],
],
'yii\bootstrap\BootstrapPluginAsset' => [
'js'=>[]
],
'yii\web\JqueryAsset' => [
'js'=>[]
],
],
],