The point of a tryout version is to allow you to try the software, in order to make you buy the software.

For some people (e.g. people who have delayed sleep-phase syndrome), a dark theme is essentially an accessibility requirement. They won't be able to try the software if it doesn't have it.

Imagine a piece of software where the demo version doesn't have screen-reader support. Would that make sense? How do you expect a blind person demo the software?