Sometimes it doesn't work, especially when it is a particularly weird colour palette, but it gets it right most of the time. In that case it does have the options to make some adjustments or just turn it of for that particular site.
Yeah, in the default settings it analyzes the CSS which will make page load appear slower. It has other options you might consider but I only have it enabled at night when I'm drowsy. (The slow page loads help slow my brain down lol.)
Ah, I gotta thank Chrome for finally making me do the change from it to Firefox when they dropped the flag for switching the web page to dark mode (when dark mode is triggered) in Android mobile... Since that moment I haven't looked back, and it seems like there is no reason to do so.
Sadly, even at the lowest brightness setting, with "extra dim" enabled, and the most intense blue blocking filter my phone will allow, most light colored backgrounds still illuminate the hell out of the room.