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Privileged pages do this automatically via: https://searchfox.org/mozilla-central/rev/aa8c75b83f636948f708986173965c84cae8c25f/widget/nsXPLookAndFeel.cpp#1179-1183 So this makes everything more consistent. Known exceptions: * Pocket panels (as those don't seem to support dark mode) * about:reader (because it has its own dark mode handling) Differential Revision: https://phabricator.services.mozilla.com/D132019