I don't understand why some people in this thread appear fixated on Win32 v. UWP and seem to be missing the obvious clue that Dark mode has already been partially implemented... It already exists in some fashion in *PRESENTATION MODE* for a note (click the Moon icon). And from my tests, it works pretty darn well which is to be expected considering it's probably doing some really basic CSS styling since notes are already HTML-based anyway.
And heck, if they're concerned that some notes won't show up correctly in some globally applied Dark Mode, then they could just take a hint from the Firefox Dark Reader extension and allow you to turn it on/off at the Note level and have the system persist the specific note preferences.
And honestly, most Windows users concerned with dark mode are most likely already configuring apps at the individual level via themes for dark mode - Pycharm, Visual Studio, Notepad++, Sublime, etc. Who cares if one app's dark mode differs thematically from another app's dark mode. I'm not decorating my house for jebus sake.