I am not sure if web developers are to blame here. It would probably be way more consistent to define default colors based on a standard instead of some local s

I am not sure if web developers are to blame here. It would probably be way more consistent to define default colors based on a standard instead of some local settings. However, if you change the colors you should make sure to do so for all related ones to avoid this. For Linux you can setup Firefox to use a different theme for content widgets which can fix assumptions developers have about the default colors. I've configured it in the about:config page like this: widget.content.gtk-theme-override;Adwaita:light