Night mode web pages in the Internet-shop of services when it is justified?

I think that this has sense only for sites daily consumption - well, that is, those where people spend every or nearly every day for long hours. For example, mail service, social networks, ebook reader, something to do with the work, Wikipedia, Google, github, Headhunter, etc.

For all other cases, the person once and there is no need to understand the intricacies of your interface. If you really really need the problem solved at the browser level (for example, UC is a function) or OS (Makosi), and not site - even assuming that these plugins often work no matter. Simply because if I go to hundreds of sites, I don't want to configure all of them individually.

And just, again, for systems with constant long hours of use can be justified. I think these sites each person no more than 3-5 pieces.

Never

If the user will want to set yourself a plugin for this

Justified only on the services for long-time reading, like bookmate.

1) the Situations are different, the buyer can visit the site with the lights in the room and then, maybe, will be not very convenient. And there are people who will be comfortable. 50 to 50, I believe. There is hardly cranks who will shop, walking home through the dark Park...
2) 1)
3) don't know
4) Once a night mode and background you can change the dark color. But the mark is not so important.