So we are using the Material theme (note Discourse differentiates themes and colour schemes, “Dark” is the latter) rather than the Default theme, so I suspect it’s part of that (and would explain why sites on the Default theme, likely many, don’t do this).
I can enable the Default theme for users, but I haven’t tested our various theme components with it, so I’m slightly wary. There’s another topic (Site-wide reduced or disabled motion settings) about reducing the visual impact of various elements of the UI, it would be good to have another data point on whether that works for people. I’ll also see about testing the Default theme too, though it might be a few days before I can get to it.