It's in beta mode and it applies to every site you visit. More than a few sites doesn't show very well. It's not a great experience yet. It definitely helps this site. A proper dark mode is very much appreciated.
Yeh, agree, it's not a silver bullet, and usually gets tripped up on images, usually buttons, that fail to be legible afterwards. But for the 5% of sites that its suboptimal, it's good elsewhere.