The `initialize-theme.js` script takes care of the following:
1. If there is a stored theme value in the localStorage, set the theme
based on that value.
2. If there is no stored theme value, check the user's system
preference (dark or light) and set the theme accordingly.
The new `main.js` takes care of the actual theme switching and listening
to system preference changes (if the user has not manually set a theme).
Now the icons are stored in CSS, and are set according to the current
theme. This allows for having different icons that dynamically switch.
Additionally, wraps social and navigation elements in ul/li.
Fixes#76
Óscar