Honestly. I am often a "dark mode" preference person, but I use lesswrong in light mode, because I think lesswrong has a light mode aesthetic and looks very good in it. I personally don't like the dark mode very much. I think it is perfectly acceptable to have light mode by default. I think that serving dark mode to users that have their system theme set to dark mode is not necessarily good. It costs you just a few seconds to change it if you don't like it, and showing the site's identity and aesthetic is good for newcomers. This is just my anecdotal experience, but it is at least evidence that not all dark system theme people prefer to have automatic dark mode.

I'd think that in this scenario the best thing to do might be to simply wait it out and ask them about it at a time that they are in less of a dominant mode. That is, maybe the next day during lunch. And then ask them, still socratically, in a way that respects their authority. So perhaps: "Hey. About the feedback I got yesterday, I've been looking to really understand it better so I can improve more. I've really been thinking about what I could and should have done, and why. Would you like to discuss it with me briefly?"

Absolutely not a professional in regards to being a good communicator, but reasoning logically, I thought that this strategy might work since it aims to avoid direct contact after the feedback (which otherwise might trigger the sentiment that you are being defensive) and also clearly shows that we value their authority and opinion, since that is why we approached them after all, thus avoiding an attack at their ego. These two together might set up the conversation in a frame in which the other party is more open to truth seeking discussion.