Browsers never stopped having that functionality. It's still entirely possible to load custom user styles into a webpage.

If I recall, Netscape or one of the other early browsers used to have an option to change the default font/color of text and the background in its settings. Nowadays you have to use an extension to do so.

I finally found an option in Firefox to change things around, but it still only changes the font and not actual foreground/background colors.