I would like that in Firefox Android when you put the dark mode all the pages (including the google pages) would be put in dark mode without having to install an extension that does it.
Or if you don't want to put it in black, modify it to your preference for example:
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Some potentially related ideas:
Built-in dark mode for web contents
Provide a black dark mode in Firefox Mobile
Firefox Android needs to add an option in settings to force dark mode on all web pages without the need for an extension.
(Note: similar ideas have been merged into this thread)