"Dark mode in reader view expands so that windows are also dark on the controls, sidebars and toolbars."
Thank god.
/* Put the content below into your <firefox profile>/chrome/userChrome.css */ .browserContainer { background-color: #000000 !important; }
(More broadly, I wish Apple would allow things like file dialogs to force to dark mode. I know you can set NSRequiresAquaSystemAppearance to NO, but it tends to mess up a bunch of other random stuff too)