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?
Well, I haven't tried the software, and won't, since I all my needs are already met and I tend to avoid proprietary software anyway.
Nevertheless, I think this is a clever strategy from Jon Skinner and his team.
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?