To those who want to change base font size of 2012 from 14px to 16px without expanding the page's width : don't change font-size property of html do it in body instead.
body { font-size: 16px; }
but only some elements will be affected, and some won't ( because they are explicitly declared its own font-size with specific class already ), so you will lose the vertical rhythm because of this.
The other way is to change font-size for all elements like what Andrew suggested, or change(re-declare) the page's width property below in media queries bit (because of the mobile-first thingy.)
Anyway, I still believe that changing base font size of 2012 from 14px to 16px would be easy, because of the power of rem, but maybe I know too little about css.