Loads a different emacs theme depending on the time of day. Uses the Emacs 24 load-theme function. In my case, I liked the sanityinc-tomorrow-* themes, so I used that. It should be trivial to adjust the theme-on-time function to load different themes at different times of day.

Adding the following to your .emacs:

;; switch themes automagically
(add-hook 'after-init-hook 'theme-on-time)
(run-at-time "00:00am" nil 'theme-on-time)
(run-at-time "06:00am" nil 'theme-on-time)
(run-at-time "06:00pm" nil 'theme-on-time)
(run-at-time "12:00pm" nil 'theme-on-time)

Will result in running theme-on-time once after the completion of the rest of the .emacs or .emacs.d/init.el. It will also run at the times indicated; different times may make more sense for different people.

Note: you cannot just run (theme-on-time) from the init file; it has to be executed after all the initialization has completed.