C# - How To Change Theme Via Web.Config On The Fly

I have a web-application with more than 5 themes. Each themes covers a completely different style for different customers. whenever we publish a new version of our application we send it for all of customers.

We specify theme in web.config file in <Page> tag. like

<page theme="Theme1" /> // or Theme2 for second customer.

with this approach we easily change the style of application from each other without writing codes which needs a new publish for each one.

With changing the theme="Theme1". nothing change and we didn't write a code in application , so why changing theme don't change the theme and we need a new publish?

we want to change it in web.config or somewhere else and with changing it the theme change without needing a new publish for each one.

Update :

I Publish Once and then copy the published version for each customer (5 times) then in each web.config file I change the theme="CustomerTheme". but only the theme which was active in publish process is usable in all of 5 versions and other 4 themes are un-usable Actually the main question is that why changes in Web.Config need a different publish while it is XML and do not need a complie. I should Add this note that, App_Theme include all of 5 themes so that all of them Compiles and are ready to use

This is the way I publish the application

Photo Link

Update 2

here IFound the exact problem reported by someone else. he resolve the problem but I can't underestand how he resolved this Issue.