PUBLIC MARKS with tags categorie & "multiples themes"

I had to create my first wordpress plugin due to necessity. One of my clients require that he be able to assign different themes for each category. I have seen this feature in the Wordpress Subdomain Plugin but I can’t use it as is since my client doesn’t want his categories to be subdomains. I looked at the code of said plugin and said to myself, “heck, this is easy.”