how to call Yii::$app frontend in backend ?

Hi, i install this plugin for change theme in frontend https://github.com/Singrana/thememanager install like manual.

in my frontend/config/main.php


'components' => [

'view'=>

        [


        'theme' =>

            [

                'class'     =>  'singrana\thememanager\components\ThemeManager',

                'current'   =>  'siteeee',


                'themes'    =>

                [

                    'site'  =>

                    [

                        'pathMap'   =>

                        [

                            '@app/views'            =>  '@frontend/themes/site',

                            '@app/views/layouts'    =>  '@frontend/themes/site/layouts',

                        ],

                        //'defaultLayout'             =>  '//inner',

                    ],


                    'siteeee'  =>

                    [

                        'pathMap'   =>

                        [

                            '@app/views'            =>  '@frontend/themes/siteeee',

                            '@app/views/layouts'    =>  '@frontend/themes/siteeee/layouts',

                        ],

                        //'defaultLayout'             =>  '//inner',

                    ],


                ],

        ],

    ],


],

but i need to call


\Yii::$app->view->theme->changeTheme('themeName');

, like in manual, to change theme , how can i do this in backend side ?