It would be cool if the background of their page changed color depending on your local time.

Liz Danzico's Bobulate[0] has had a "night mode" for years, based on your local time-zone:

[0]: http://bobulate.com/

My fix is to not use dark themes anywhere. When I'm on a website with a dark background I click my "zap colors" bookmarklet which reverses the colors: https://www.squarefree.com/bookmarklets/zap.html

How about dark everything? I give you Hacker Vision: https://chrome.google.com/webstore/detail/hacker-vision/fomm...

This is an important problem to fix, because the eyes adjust to the dark screen (your pupil opens up), and in fact switching quickly to a bright screen is extremely alerting.

I used to use black terminals, then went to white due to most webpages being white. Switch from black terminal/emacs to a white webpage was just too irritating. (sometimes painful)