My sense is that theming like this is generally not aimed at people who would use a Mac in the first place, but rather BSD users who want something that looks m

My sense is that theming like this is generally not aimed at people who would use a Mac in the first place, but rather BSD users who want something that looks more pleasant than their default window manager. Creating a coherent visual language from scratch is a huge undertaking, and requires a certain skill set. Mimicking an existing design system is much more straightforward, even if the results are uneven.