I did the original conversion of the text into HTML and it was certainly before 1999 and I'm pretty sure before erowid existed. One of the first resources for this kind of thing on the internet was an ftp site that I ran in 1991 or so. Eventually Brian Behlendorf set up hyperreal.org and I think at some point all the information was fed into that, and then later that got all folded into erowid.
Dark mode was actually pretty common in the mid-90s, then some idiot popularized the notion that black text on white background was easier on the eyes and more familiar for non-computer users and that nonsense continued for about 20 years (old CRTs were all white/yellow/green on black, stuff like xterminal displays often emulated their color schemes, it seemed like windows and mac was mostly where the white background was coming from -- lots of websites were the plain grey default background initially, but I remember black backgrounds being much more popular in the early web until they were nearly run out of existence).
Oh wow, the archive.org site is the original HTML that I did. Yeah, so 1999 was probably when I flipped it over to erowid after leaving UW or something like that.