Some types of application I prefer light, some dark. But the premise is wrong:

Saves your battery

Only if you're using OLED, which most people aren't, and by very little. You could save more battery by using a low-power mode when you don't need all the bells and whistles.

Easy on eyes

Debatable, different for different people.

Dark theme is just nicer

That's not objective, it's personal taste :)

It's not even that. Applications should ask the desktop environment to present information, and not need to know about your colour choices. There's no reason to have separate "modes" in different applications.

So it's a like-for-like replacement of Twitter then.

1Password has some nice features (like it reads QR codes off the page and automatically handles 2FA for you, which is clever, but not necessarily the "2" in "2FA" you were hoping for) but it also has a lot of weird UI decisions that make it confusing to use, especially in a shared company environment.

It is a lot better than it was before though, now it's cross-platform (it used to be exclusively AppleSuperiorityComplexWare), but it's still not open source.

In my experience which is worth one (1) datapoint, people talk about all sorts of stuff.

Yes, people do talk about the fediverse as part of it, but usually it's about how we can make it better rather than how our corporate overlords have made it worse.

And I'd say that fediverse metacontent is less than 1% of my feed, so it's not universal, it's very much down to how you set things up.

Places like kbin will get more because they're newer. I'm not on Threads or Blue sky or whatever but I imagine a lot of the chat there is about their own shiny new thing as well.

Where does the 18 come from? Depending on the content and the location, "adult" would be more useful, wouldn't it?