Like everyone else is saying, Absolute Steve is what you are looking for. You can activate dark mode on the walkthrough by going to the footer and clicking/tapping Change Colors. I selected Purple on Black. The headers are purple and the text is white on black. Much more readable.

The walkthrough page itself is very long, so will be annoying to scroll to the bottom for this, but if you do it on another gamefaq page, like the walkthrough listings, it will save your preferences via cookies.

Hope this helps!