Keep the old theme "smoothness" as the Yii default for backwards compatbilitiy, based on discussion in https://github.com/yiisoft/yii/pull/4344

UPGRADE

@@ -23,17 +23,17 @@ Upgrading from v1.1.23
 
 - jQueryUI was updated to 1.12 and contains possible breaking changes.
 
-  The default base theme changed to a more modern theme with some colors, less gradients and some other improvements.
+  Upstream the base theme changed to a more modern theme with some colors, less gradients and some other improvements.
+  The old default theme "Smoothness" is still included and configured as the Yii default for backwards compatibility.
   
-  The old default theme "Smoothness" is still available if you wish to keep it.
-  You can apply it using the CWidgetFactory application component for example:
+  If you wish, you can apply the new base theme using the CWidgetFactory application component for example:
 
       return [
           'components' => [
               'widgetFactory' => [
                   'widgets' => [
                       'CJuiDatePicker' => [ // list your widgets derived from CJuiWidget here
-                          'theme' => 'smoothness',
+                          'theme' => 'base',
                       ],
                   ],
               ],

framework/zii/widgets/jui/CJuiWidget.php

@@ -38,10 +38,10 @@ abstract class CJuiWidget extends CWidget
 	 */
 	public $themeUrl;
 	/**
-	 * @var string the JUI theme name. Defaults to 'base'. Make sure that under {@link themeUrl} there
+	 * @var string the JUI theme name. Defaults to 'smoothness'. Make sure that under {@link themeUrl} there
 	 * is a directory whose name is the same as this property value (case-sensitive).
 	 */
-	public $theme='base';
+	public $theme='smoothness';
 	/**
 	 * @var mixed the main JUI JavaScript file. Defaults to 'jquery-ui.min.js'.
 	 * Note the file must exist under the URL specified by {@link scriptUrl}.