Since f.slux can change system theme to dark mode after sunset, why not do the same to wallpaper?
There are some nice new wallpapers in El Capitan that could work well with this;)
On that note, how about we figure out a way to invert all webpages that have white backgrounds!
I've used this on Chrome:
there were some other tools that could make the sites high-contrast, etc.
Unfortunately I didn't see nothing like that for Safari (which I use now)...