The web site uses Twitter Bootstrap. The easiest way to customize the CSS is to customize the generation. This is a list of the custom settings, make sure to update it when madifying something else.
Typography
@baseFontSize: 18px
@baseLineHeight: 24px
Run it locally
jekyll serve --watch
if you have newer Jekyll.
Requirements
You need a version of Ruby. ruby -v to see whether it's installed.
gem install jekyll