The icons are black, or rather, default fill (black), which would probably explain why they show up as, well, black. Based on the echo icons, which also appear to have unspecified and thus default fill, the only difference I can see is that the echo icons appear to be embedded, the timeless ones are not?

If that's really the problem here, I'd say opera is being rather sad and pathetic, but the icons probably should be embedded regardless in order to reduce requests...

It could definitely be a case that Opera is doing the wrong thing. Not that I think there is a 'standard' for this stuff. I can report the issue to them too, but I'd think it would be helpful to understand the problem a bit more first.

Are there other browsers with night/dark mode support? I checked Firefox and Chrome but didn't see anything like that.

There is Dark Reader addon for Firefox on Android, not so sure about Chrome.

Granted this is not a native feature but still a way for dark/night mode.