Normalize body font size using percentage instead

TOC section is not displaying well in Firefox as the text is obscured by the background image.
![Screenshot in Firefox](https://f.cloud.github.com/assets/1570336/285806/0d9e8678-922a-11e2-8947-029b6a31aa48.png)


Problem due to different interpretations of font sizes, and thus ems, in different browsers.