Well, given a choice, I'd prefer the users not get to select forums of their choice if it meant I could change the themes for categories. It is far more important for my site (and many sites like it) to be able to customize the themes for multiple categories/forums than to allow users to create their own "prefered viewing" list.
After all, the option for users to select their own forums to view removes the need for Categories so if that option is created, it should require that a whole seperate script be used for displaying the forums anyway. Thus avoiding any conflict. These "custom" user views could by defult use the forums default theme.
