Single Post Template – per category

I am trying to be able to make a category be kinda a micro site but i need to have it a separate theme, i already know how to make the page and category theme. but when you click on a post to read it it goes to the sites default theme, Is there a way i can set it so all posts in “category” use a different theme?

Please let me know if you have any ideas, it seems odd that you are able to make custom pages and categories but not post templates.

Thanks