Windows 10's dark theme basically has this problem: it makes everything #000000 black, which is not most people usually want. It's really tiring to the eyes and I switched back to the light theme instantly.
I generally avoid dark themes because 1. they are IMHO very ugly, 2. I find that "night mode" apps like redshift are much better for tired eyes, and 3. I noticed that black themes users often end up keeping a higher screen brightness level than me, which counters IMHO any benefit you may ever gain from night mode.
I don't understand how anyone can read white on black without their eyes hurting.
When I first tried it out I was instantly taken aback at how MS had used full black for their darkmode. Not suprising, considering their inability to implement reasonable design, but ridiculous nonetheless. I can't fathom how no one on their design team stepped back and thought 'maybe there's a reason dark mode is usually a dark grey instead of black'.
It is most likely due to AMOLED and their big investment in mobile technology (Surface, etc.) I imagine a lot of them have AMOLED screens, so a pure-black dark mode pays off here. Nevertheless, I can't imagine it would be hard to just let people choose the colour of the dark variant, like GTK / Qt theming systems do on Linux.
Not designing for a variety of displays and viewing conditions is frowned upon in general, but should be inconceivable for a company the size of any MS org.
On that note, this is one of the reasons I still use a 1x display as my main. Currently it’s a comically large 43” 4k@1x, so it’s only representative of “normal” usage when I remember to shrink windows, but still.