Sure but they probably won't offer that much. What I mean is that the value of this app, despite being free, is not zero, but that's probably what the developer was getting. So even if he got 10K for it, he's happy and moved on.
The app originally set the desktop theme to dark mode or not. That’s not worth more than a single one dollar payment, if that.
If I want to do some shady app shit I am buying install base. 100x an app that makes a few grand a year could still be worth it for my nefarious purposes.