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.