Given a location and day/night color themes, this file provides a change-theme function that selects the appropriate theme based on whether it is day or night. It will continue to change themes at sunrise and sunset. To install:

Set the location:

(setq calendar-location-name "Dallas, TX") 
(setq calendar-latitude 32.85)
(setq calendar-longitude -96.85)

Specify the day and night themes:

(require 'theme-changer)
(change-theme 'solarized-light 'solarized-dark)

You can also pass nil as either of parameters to change-theme, with the effect of not using a theme (or using the default Emacs theme) during that period of the day. For example:

(change-theme nil 'solarized-dark)

will result in setting the default Emacs theme during the day, and solarized-dark during the night.

Note: you may need to add the repository path to your loadpath. For example:

(add-to-list 'load-path "~/.emacs.d/elisp/theme-changer")

If you want to use the color-theme package instead of the Emacs 24 color theme facility:

(setq theme-changer-mode "color-theme")
(change-theme 'color-theme-solarized-light 'color-theme-solarized-dark)