I know that you can have different themes for different parts of the site, assigning specific URLs to different themes by using hook_custom_theme() or the themekey module - I've used both solutions to do this at one time or another.
I'm asking because I want to A/B test an upcoming UI fix. In my test, 50% of visitors in the control group would see "old pages using theme A" (without fix) and the other 50% would see "new pages with theme B" (with fix). Then I can see whether the fix improves performance/conversion and by how much.
The thing is the testing tool I use needs to have a way to change the theme for that latter group only. That tool (Optimizely) doesn't have access to the PHP hooks and functions, but it can alter the on-page JS code for the control/treatment groups.
Any advice would be much appreciated. If this is a bit confusing, I can clarify any questions ...