First off, I did look through the man pages for the answer to this. Sorry if I missed it.
I’ve seen a few builds of fvwm that manage to change the wallpaper from page to page within a single desktop. Just how much can you change from page to page? Is it possible to have a totally different theme? Window decorations? Button bindings? What about between desktops? If I had three different desktops in a single fvwm session, could they all have three different themes?
As for window-themes and the like, you would have to pre-declare your different decors with “AddToDecor”, and then use FvwmEvent, listening on new_{page,desk} (depending on your needs) to style all windows on that page/desk with what you wanted.