Set Light and Dark themes based on time of day (or system)

[dessalines]

It'd be really helpful to have some kind of a theme-mode switcher, based on the time of day.

On vim, I was able to set startup themes using vimscript:

if strftime("%H") > 17 || strftime("%H") < 7  
  colo monokai
else
  set background=light
  colo PaperColor
endif

This wasn't ideal, because it only ran on startup.

[the-mikedavis]

#2158