avoid rem in media queries

[mbostock]

Ref. https://www.w3.org/TR/mediaqueries-4/#units

Relative length units in media queries are based on the initial value, which means that units are never based on results of declarations. For example, in HTML, the em unit is relative to the initial value of font-size, defined by the user agent or the user’s preferences, not any styling on the page.

So instead of rem this now uses multiples of 17px which is our equivalent.