updated for JSS version 7.0.3

jss-important.js

import { endsWith, has, forEach, isString, isNumber } from 'lodash'
import cloneStyle from 'jss/lib/utils/cloneStyle'
const IMPORTANT = '!important'

function makeImportant(value) {
  return endsWith(value, IMPORTANT) ? value : value + ' ' + IMPORTANT
}

export default ({important: importantByDefault = true} = {}) => {
  return {
    onProcessStyle: (style, rule, sheet) => {
      let important = importantByDefault

      if (has(style, 'important')) {
        important = style.important

        delete style.important
      } else if (has(sheet, ['options', 'important'])) {
        important = sheet.options.important
      }

      if (important) {
        let _style = cloneStyle(style)

        forEach(style, (value, key) => {
          if (isString(value) || isNumber(value)) {
            _style[key] = makeImportant(value)
          }
        })

        return _style
      }
    }
  }
}