It is difficult to perform user dependent theming through a general configuration in 'main.php'.
This article proposes a method to do so through a Factory that dynamically generates classes to allow delayed resolution of actual user values.
This means that you can refer to values that are stored in the database through the CActiveRecord before Yii is actually loaded.
The values will be fetched from the database when actually used for the first time, which is after Yii is loaded.
Actually, you can use pretty much any method that relies on Yii with this method.
There is an issue on Yii2 to redirecting / urls to /frontend/web.
In other words we need a way to hidden /frontend/web from addresses.
We will do this without changing Apache configuration and creating virtual host or setting document root (It's good for share hostings that we have not access to apache.conf)