I completely missed the icon to begin with as it doesn't stand out well enough and I am trained by other sites to expect the icon to be more visible.
There are obviously different ways to go about it but the coin market cap one is an example of a simple way to do it that works and looks good. In their case they just used black and white, with a bold style.
I get your point, we'll see how we can make it more visible.
This is a very elegant idea. On the other hand I don't find myself changing this setting regularly. I use dark mode for everything. I would be fine to have the setting in user preferences section of the menu.