I think that they should mostly avoid CSS when needed, and then it should use the default colours configured by the user. But when they need to specify colours

I think that they should mostly avoid CSS when needed, and then it should use the default colours configured by the user. But when they need to specify colours in their document, then they should specify all of them (specifying only some colours and not others causes problems) and use the "prefers-color-scheme" command to implement it. It is then the user's job if they like dark mode or not. If you do not like dark mode, then do not enable it in your web browser.