I found in the past using AutoCAD and 3DS Max that dark mode was easier on the eyes when working long hours on the screen. Even with today’s LED/ OLED options versus CRTs of the past which were very harsh on the eyes, I think dark mode is a great option to have the ability to turn on and off depending on where, what and how long you are working on something.
Also, as I’ve aged, I’ve noticed more floaters in my vision that are more visible with light backgrounds (ex: a white field or a clear blue sky), than they are with dark backgrounds. This causes my brain to have to work harder filtering out and ignoring the floaters which cause a bit more fatigue.
I’m always a fan of given users options to work the way they like, and I think a dark mode toggle would be a relatively easy low-hanging-fruit kind of an option to implement that would make enough SketchUp users happy including myself.