There are two themes you can select for this web site, plus you can select to
use this web site with the default style provided by the browser.
When a theme has been selected, that theme will be used for all other pages
of this web site and will also be used in subsequent visits from the same
client without using cookies.
Technically speaking, it uses the HTML5
Web Storage API
to store the theme. (Also see Web Storage (Second Edition);
a sample is at HTML5 localStorage (name/value item pairs) demo).