message: Adds some constants and a media query mixin
diff: +import { css } from 'styled-components';
  +
  +const SIZES = {
  + mobile: 640,
  +};
  +
  +const media = Object.keys(SIZES)
  + .reduce((acc, label) => {
  + acc[label] = (...args) => css`
  + @media (max-width: ${SIZES[label] / 16}em) {
  + ${css(...args)}
  + }
  + `;
  +
  + return acc;
  + }, {});
  +
  export default {
  emptySpaceWidth: '0.33em',
  headerHeight: '50px',
  + blockWidth: '315px', // TODO: 19.5em
  + blockGutter: '10px', // TODO: 1em
  + media,
  };
  