The Theme buttons are dymanically generated based on the the link (or style) tag's title attribute. rel="stylesheet" is the preferred, enabled style sheet, and all style sheets with rel="alternate stylesheet" are disabled. Clicking the buttons toggles which style sheet is enabled. Any style sheet without a title attribute is persistent (always enabled). This is based on the W3C spec: http://www.w3.org/TR/html401/present/styles.html#h-14.3.2
This form of style switching is more effective than changing an attribute in the body tag. It makes coding CSS easier, because less emphasis is placed on targeting through the body tag class, and all CSS theme's could theoretically have the exact same selector names.