I ended up setting up custom themes for multiple different widget sets to get a true black background. It was easy for most QT variants, not too bad for GTK2, really awful for GTK3 because it doesn't have proper documentation for manual theme creation, and I haven't tried to tackle GTK4 yet.
Because they all need different configs (and the window manager title bar etc. may need yet another one), it's difficult to give suggestions unless you tell us which terminal and window manager software you're trying to theme—the requirements for a Gnome session are different from those for something like fluxbox. Some terminal software even has its own built-in theming support.
Most people are not obsessed monomaniacs with room for only one interest in their lives. That means that most Linux users are interested in things other than free software, so many will choose to "dress up" their computers in ways that showcase their other interests, and may use material that is not under copyleft licenses to do so. If this causes you anxiety or confusion, you may want to speak to a mental health professional. Seriously.