I see where I can enable dark mode for Yahoo Mail, Yahoo Finance, and Yahoo Fantasy.
I found a post that talked about a settings icon on the Yahoo home page, and either that article was wrong, or the settings icon is not easily discovered at all - nor does the Yahoo Help indicate that there is such an option.
I'm heavily considering using another home page than Yahoo, since it's the only one that I cannot get working in a Dark Mode and it's glaring white pages strain my eyes.