I'm pretty sure I already know the answer to this, but thought I'd ask anyways since I wasn't able to Google an answer...

 

Is there a way to switch between different themes using a script? Specifically without duplicating a layout, changing the theme, and switching between the two layouts?

Your suspicions were correct - you cannot change theme via script.  Scripts work at the browse level and themes are layout-level.  I sure wish we could!!  :-)

In addition, it is best to keep the number of themes to minimum (1 maybe 2).  When you use a theme, all elements are loaded whether they are used or not.

I suppose you could use conditional formatting and globals, and cut a paste layout elements all over the shop, but I suppose that's exactly what you were trying to avoid...

Yup, I'm trying to avoid duplication wherever possible. It's good to know about having multiple themes - I'll keep it to one primary theme for simplicity. Thank you for the responses.