There's an ok in-between where wide windows get margins and narrow ones use the full width. But yes a lot of sites have the stupidly huge margins.

The best alternative is a site that is responsive that takes advantage of wider screens but not lazy fullwidth.

Why is full width lazy? It uses all your screenspace;)

Because it forces your users to make your site readable which is lazily passing off the responsibility.

And what if users want light or dark theme too. How do you guess it?

You don't have to guess. You can just use https://developer.mozilla.org/en-US/docs/Web/CSS/@media/pref...

How it plays with the idea of not passing responsibility to the user? User can request window width too.

Making text wider is lazy. At some point, you'll need to break it into columns so that the text is readable. Perhaps on super wide screens, three or four columns (akin to a two column book that's wide open or a newspaper) may even be more preferable.

Otherwise, the other option is to scale up thr text size, but that takes up precious vertical space on wide screens, and doesn't increase information density.

Obviously, text columns may only make sense it certain scenarios - a minimum amount of text might be required to justify a second column as opposed to just limiting the width of the first column.

No its not, there are lots of more alternatives.