This is awesome because I like reasonable solutions that let me be lazy.
I'm sure there are designers that love dealing with two themes, and will do beautiful work within each theme.
But I'm really fine with letting the computer figure it out. CRM, back office tools, admin dashboards... many clients are approving one design and dark mode is an afterthought. Why can't the computer just... do it for me, and do a better than passable job at that.
Modern CSS has some simpler mechanisms to deal with the light/dark dichotomy now. I translated a blog to support dark mode recently and it wasn't that difficult. The two sticking points were <code> and picking foreground colors that didn't blend in, and the github icon.
The other was Github's icon. But `filter: invert(1)` does a pretty decent job of handling that as long as the icons are black and white.