I don’t fully understand typography’s vertical-rhythm, but here are my thoughts.
Rem units (“root em” applied to the html element) are used instead of em units, to avoid tracking an elements hierarchical context as how a user will “nest” elements in a CMS cannot be predicted.
The pixel units are provided for a fallback for older browser that do not support rem units . The pixel units are written first in the CSS “cascade” so newer browsers that support rem overwrite the pixel units.
The baseline rem units is normally equivalent to about 16px for what I have read. So I was at first puzzled why the theme was using 14px ($rembase). It appears that html element is then styled with 87.5% , instead of 100% (the ration of 14 to 16). I think this is done to make maintenance of vertical rhythm easier by applying the theme’s suggested top and bottom margins of 24px or 48px (vs 27.428571429px and 54.857142857px).
The precision several decimal places out may be due to CSS preprocessing using something like LESS, SASS or Stylus. Browsers are capable of reading this precision. Best practices I have read suggest not rounding these values.
