is the Mojave Dark mode available even for WIN?
No, that is Mac-only.
no plans for it?
Not that I’m aware of, no.
Is this still the case?
Windows 10 has added official third party support for dark mode as well now, and for example Chrome seems to be one of the first apps to take advantage of it.
Hi - yes, this is still the case.
Rhino 6 for Macintosh has dark mode. You can see it in 4:34 to 5:06 time of this video: https://www.youtube.com/watch?v=iFU-iMU1G8c
Adobe Flash problems forced McNeel & Associates to replace Rhino help files with HTML files. All these HTML files have white background. I prefer black background. Some Internet browsers, including Chrome and Firefox, support black background.