Your hunch is incorrect. Perhaps they have removed this functionality, I haven't checked, but this historically is functionality that was used to change the default background/text colors of all pages, built in to the browser.
Hm... Perhaps you're misremembering? I'll be fairly surprised if this was ever in Chrome, for a few reasons. When do you know that this last existed?
Closest thing I can find are custom user stylesheets – which were removed in 2014 (also not sure if "write some CSS" is an a11y solution, heh): https://codereview.chromium.org/66383005/
I know at least firefox at one point used the system default background and text colors. This of course worked horribly as many websites (including Google at some point) would override the background or text color with a custom value and not the other assuming that the user would be using the default theme on Windows (or just not caring), which left you with dark grey text on dark background on many websites if you used a dark system theme.
I do agree the the actual point that the default colors have nothing to do with user preference though.
