Back when Bootstrap 2 was the most popular UI library one of the biggest problems with it was that the defaults were good enough that no one changed them, and consequently every SaaS startup's product looked the same for about 5 years. Pushing people to tweak things while still giving them a solid foundation is a good thing.

I'm not saying that's necessarily the case here, but there is a strong argument in favor of not making the default theme for a UI library look perfect.