My app calls Color.IsDarkMode for custom controls and otherwise uses Color.TextColor etc. to use system colours where appropriate. It responds correctly to Dark Mode being turned on or off on my machine running Catalina.
However, a user has just reported that on his Catalina machine with General Appearance setting on Auto using Night Shift, his desktop is not in Dark Mode, but the App is displaying in Dark Mode.
Oh, wait a minute … since it sounded unbelievable again when I typed it out, I just checked his video again. His System menubar is clearly displaying white text on a dark background. That is Dark Mode. So this is just user error, looks like he’s confused by his system setting of “Auto”.
His system was switching between modes not according to time (which would be predictable) but according to ambient lighting. That’s why he was confused.