function css(strings, ...keys) {
  const lastIndex = strings.length - 1
  return (
    strings.slice(0, lastIndex).reduce((p, s, i) => p + s + keys[i], ``) +
    strings[lastIndex]
  )
}
Converts a string of CSS into object syntax
@param strings
@param keys
@returns {Object} CSS in object syntax
@example
const output = css`
 html {
   color: rebeccapurple;
 }
`