After looking for 5 days I fixed it on my end. I had changed a bunch of stuff trying to get rid of the white glow, but these steps finally fixed it for me.

Go to opera://flags/#enable-force-dark and put it on Default. Go to opera settings, at the bottom-ish, there's a setting called Force Dark Pages. Turn that off. Then go to youtube and turn on dark mode on youtube itself. Restart the browser and that fixed it for me