Scale UI components with font-size using em units, remove hardcoded px units

[benjamingwynn]

The font-size is currently hardcoded for a lot of styles. As 98.css isn't readable at high resolutions (see #56) it would be useful to be able to define an explicit font size and have components scale correctly accordingly.

For example, instead of the font-size: 12px hardcoded on the body and font-size: 11px declared for button, label, input, legend [...] we can define a font size of 0.91667em (11/12 ~= 0.91667) and remove the font-size on body.

I have a prototype of this but not with all the sizes yet, I'm happy to finish it, write up a PR and make it official if this is useful!