

I don’t think privacy related browsers will ever really become mainstream. The closest we have is probably Brave, and more people honestly use that because of hype and adblocking still working post manifest v3 on it while still being chromium based.
I’m honestly very happy with Librewolf, but then again I default to browsing the web with JS turned off, so I’m definitely not anyone’s target audience.
You can accomplish this using userContent.css stylings.
Dark mode for the pdf itself can be accomplished by following this blog guide.
Once set up, you can then paste additional stylings to get the background and sidebar whatever color you prefer. Here is my userContent.css.
The commented out lines towards the bottom are for the sidebar (I personally didn’t change the sidebar).
Hope this helps.