It is absolutely ridiculous that, in 1995, I could completely change the font sizes and color scheme of my UI radically and it would work with the vast majority of applications, but in 2019 I have to wait for a developer to decide to add a theme or provide some kind of customization facility unique to their software.
In case people don't believe this, there was an update for Office THIS WEEK for Outlook to support Dark Mode on OSX Mojave.
Nowadays, we expect the same services to work on our laptop, our work computer, our personal iPhone, our work Android, our smart TV... the importance of parity between versions is much higher.
To me, dark mode is the prime example of the flimsiness of relying on native GUI solutions...for years Apple ignored users and apps that wanted dark guis, despite the fact that dark mode is legitimately a lot better for a lot of people
We could tune up our own universally applied dark mode in 95 quite simply, but now each application has to implement that itself.