From a user-perspective I think it's not suitable to switch between themes only to make one application look different — I don't think that anyone would follow that advice.
Also, for users who enabled the native Dark Mode option of Windows 10 (via "Settings > Personalization > Colors > Dark") I can understand why it would be nice that as many applications as possible would support it. It feels less like an accessibility mode (the high-contrast mode does) and more like a personal choice that is less straining on the eyes (for some users).
Unfortunately, there is no official API for Dark Mode yet. I did some preliminary experiments hacking around this, but gave up after realizing how much work this would be.
I'll keep it on the list as a feature request and might come back to it in the future.