backend/views/layouts/main.php

<?php
/* @var $this \yii\web\View */
/* @var $content string */
use backend\assets\MyAsset;
use yii\helpers\Html;
MyAsset::register($this);
?>
<?php $this->beginPage() ?>
<!DOCTYPE html><html lang="<?= Yii::$app->language ?>" ng-app="BackendApp" >
<head>
    <meta charset="<?= Yii::$app->charset ?>">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <?= Html::csrfMetaTags() ?>
    <title><?= Html::encode($this->title) ?></title>
    <?php $this->head() ?>
    <!-- HTML5 Shim and Respond.js IE8 support of HTML5 elements and media queries -->
    <!-- WARNING: Respond.js doesn't work if you view the page via file:// -->
    <!--[if lt IE 9]>
    <script src="https://oss.maxcdn.com/html5shiv/3.7.2/html5shiv.min.js"></script>
    <script src="https://oss.maxcdn.com/respond/1.4.2/respond.min.js"></script>
    <![endif]-->
</head>
<body class="skin-blue sidebar-mini">
<?php $this->beginBody() ?>
    <?= $content ?>
<?php $this->endBody() ?>
</body>
</html>
<?php $this->endPage() ?>