I don't quite understand this "brighter makes you happier" thing. I'm sure there's some background to it, but in IT, I don't think it has significance. We're already in twilight here, I just installed Opera Air for the first time and it almost burned out my brain 😃
For me, a bright display is often more irritating, while a darker one is more relaxing. This is especially relevant if you're using a stronger monitor, although even if I turn down the brightness, I still have to squint at bright displays.
I'm really looking forward to dark mode because it seems to be a good browser overall 🙂