I saw a light mode user and I didn't tell him to turn on dark mode.
Why didn't you want to show him dark mode?
Because he used light mode several times today
Well, that probably means he is used to it and wants to continue to use it. So you are fine with it. One day if he asks you can just tell him about it.
He still looked like an idiot.
He is probably someone who is a slow learner. Those types are all over the world. One day he will wise up though.