default dark themes make surfing less secure
Not less secure per se, but less anonymous. Default dark mode reports your preference to websites and analytics, so it ends up being something that makes you different.
Same reason privacy browsers use a default resolution and won’t let you stretch websites bigger if you have a huge monitor, keeping a border instead…
The idea is to devalue tracking attempts by making the results a big nothing burger of more of the same. A herd of clones.
Fair, and this is why I hesitate to recommend LibreWolf as a true alternative to Firefox. It wasn’t meant to be the common man’s general browser. It was flavored with a privacy goal in mind and that’s what it does.
It’s a half-step toward recommending someone use Tor browser as a daily driver. Which wouldn’t work out very well. But it’s quite private and anonymous!
There just wasn’t a need/enough funding or drive for another “good enough” browser like Firefox, the answer for that was Firefox. Everyone else is off one deep end or another.