Well, let’s take the word “theme” out of the picture and just say different design. Didn’t Kafkaesque create a plugin that would do something similar with categories. That’s a starting point for a roll your own approach.
Maybe the best thing to do is figure out which pages of your SITE are going to be using which design. Say you will have 10 pages total, and four use design A and six use design B. Make design B the “default” design.
Then for the four pages using design A, you should make sure that the stylesheet for that design contains ALL of the images and graphics and everything those pages will use. So that no design element is contained in the content pages. (Which is what we all should be doing anyway.) If you can set your site pages up that way, can you just call that design A stylesheet in the head of each of those pages or will it step all over the rest of the WP pages?
I’m not familiar enough with the backend of WP to know whether this is doable but if someone can conjure it up, then I’m sure someone out there will be able to figure out a way to execute it. I have faith in you all. ??
And OP good luck with your concept. I am wondering tho why such a marked departure design-wise? Don’t you want cohesiveness in your site? Wouldn’t a slight difference in design achieve the effect you want? I’m just playing Devil’s Advocate here.