CSS zen garden worked because you had people writing different styles for a single HTML document. How would you write styles for every site? Sure CSS does colors, but it also does a lot of other stuff that isn't easily transferrable between websites.

HTML that uses sane markup, ids and classes goes along way towards making CSS skinnable.

They are cool, but I do think there is value to class-light pages.

There are extensions like dark reader and I use them.

But you cannot just flip colors - pictures for example have to fit their background, for not breaking style.

Me too, although I use bookmarklets instead of extensions.

A bookmarklet can change images too.

We would just have to decide on an algorigthm for the images.

For example here on HN - the algo could detect the similarity between the orange in the title bar and the orange in the Logo. And change the logo accordingly.

Or the site could add some semantic info on how the colors should behave.

I dunno, seems kinda weird we've decided that Light Mode and Dark Mode are the Two Themes and every website must be made to support them.

Why can't we just have full on theme support? We've had this before on many websites.

Or even an OS-level feature like it currently is on MacOS. It is very convenient to change the mode globally and have all your apps, including browser, IDE, email, etc. change as well.