https://old.reddit.com/r/firefox/comments/181atxy/tip_change_default_page_background_to_black/

This has someone that says that they can use toolkit.legacyUserProfileCustomizations.stylesheets to do it.

Might try opening an unstyled, "hello world" webpage to confirm that it doesn't leave it unreadable or something. They're changing the background color but not the text color.