Thanks for all the quick responses!
I fixed the double coding, thanks for noticing that. And it is working…to an extent. It makes the text box black, but not completely. There’s still a white blob between the black background and text box that isn’t affected (maybe WordPress doesn’t consider part of the page?). I tried that code NeoTechnomad, but didn’t seem to do anything differently.
Out of curiosity, I’ve heard there’s a version of the custom CSS editor that you pay for (and isn’t a plugin). Does it make a difference which you use? And thanks amaya. 🙂