Providing both is not a complicated thing, especially with support in CSS, browsers, and frameworks. That is, unless one designs websites that are so complex and buggy already that supporting a second color scheme costs "a lot of engineering time".

I think I disagree.

Just correctly executing on color scheme is a lot of work. It seems obvious that 2 themes will be 2x the work.

There is a lot of images that will require 2x the work if you want to it correctly. Let’s say screenshots of an UI in a tutorial.