Fun fact: even though author have expressed a lot of opinions in their CSS, still for me the result is probably different from what they expected: since author does not define font colour, I see it in my default (not black but dark brown), plus paragraphs are slightly indented for me, since I limit their maximum width through userstyle. Funnily enough, background colour that author chose (and set, #F5F5F0) is nearly identical to my preferred background colour.

Authors defining background colour without defining text colour is the reason we cannot effectively have default dark background and light text default in our UAs, because in this case we would see authors (light) background and ours (light) text.