You are correct. There was no OOTB dark mode in 1998. I remember it well because I wasted some time every now and then trying to make one using the appearance control panel. It was frustrating because I could come very close to a universal dark mode. But I remember certain types of text couldn't have their color independently set. One of them (body text? can't remember) used the same color as title bars. Which would make it unusable in a proper dark mode.

I guess I was slightly ahead of my time in wanting system wide dark mode. XP with the theming hacks made me very happy.