No idea what's wrong with your face unlock, none of us has that issue.
As for Dark mode, it has been said many times already and is clearly written in the sticky post in Bugs subforum: Dark mode is still in development by Xiaomi, it is incomplete, and telling us that it is incomplete/buggy does nothing more than waste our time reading through those comments.
Moreover, it is very likely that by the time Dark mode is fully ready (probably MIUI 11?), your device might be EOL or dropped Xiaomi.eu support. I find it unlikely that Android Nougat gets a fully implemented Dark mode at all.