packages/jss-plugin-extend/src/index.js
function extend(style, rule, sheet, newStyle = {}) {
  mergeExtend(style, rule, sheet, newStyle)
  mergeRest(style, rule, sheet, newStyle)
  return newStyle
}
function extend(style, rule, sheet, newStyle = {}) {
  mergeExtend(style, rule, sheet, newStyle)
  mergeRest(style, rule, sheet, newStyle)
  return newStyle
}
Recursively extend styles.