There should simply be a button -- a conspicuous one -- that toggles the color scheme. It's trivial to add such a button. It doesn't need to be tied to a user ID; it doesn't even need to set a cookie. The fact that no such button exists is a choice someone made, a poor choice that disregards decades of human-machine interface research.
Failure to go full Karen about goofy things like this has made the Web a little worse for almost everyone in one way or another. So... there ya go.
Is it possible that your instance moved it away from the default place? What instance are you on, I can help you find what you need to click.
Otherwise it's up to the instance admin to choose the default.
