Fixes #5360: Added back BootstrapThemeAsset.

extensions/bootstrap/BootstrapThemeAsset.php

+<?php
+/**
+ * @link http://www.yiiframework.com/
+ * @copyright Copyright (c) 2008 Yii Software LLC
+ * @license http://www.yiiframework.com/license/
+ */
+
+namespace yii\bootstrap;
+
+use yii\web\AssetBundle;
+
+/**
+ * Asset bundle for the Twitter bootstrap default theme.
+ *
+ * @author Alexander Makarov <sam@rmcreative.ru>
+ * @since 2.0
+ */
+class BootstrapThemeAsset extends AssetBundle
+{
+    public $sourcePath = '@bower/bootstrap/dist';
+    public $css = [
+        'css/bootstrap-theme.css',
+    ];
+    public $depends = [
+        'yii\bootstrap\BootstrapAsset',
+    ];
+}