I am applying an answer in ASP.Internet MVC that later does apply to handful of other fields. To do this it may need to re-brand the UI despite the fact that the actual business logic wont have to change. Let me write the code in a way that will permit other designers to simply develop code which will only changes the UI. This is comparable to the way in which styles could be written against Wordpress Blog software.

Can anyone suggest how you can organize my project to create such feature work?

I wouldn't use bult in Styles (not really confident that these remain in MVC) However, you could multiple teams of CSS (with related images) inside a Styles folder having a separate path per theme eg: ThemesDefault, ThemesClassic, etc in which the only configuration may be the Path element. This could split the styling in the core code and also you would'nt want to use any Theme "Engines" etc.

Inside your MasterPages/Pages/Sights you can just set the road to the stylesheets dynamically.

Perhaps you have attempted using MasterPages?