Does the author aware the blog is unreadable on Firefox mobile? I might have something wrong on my phone too, but I don't have an issue with anything else.. https://imgur.com/a/ctriVnD
That's the "drunk" theme the site has. Scroll all the way to the right in the switcher, and select "reset".
Did you muck with the system fonts? On my Android FF it looks just fine, and yellow not black.
You have pressed the "Drunk Mode" button on the theme switcher - probably on a previous visit.
Change the theme at the top and normality will be restored.