multifact-pivottable.js negative number format

I’m using the multifact-pivottable.js/PivotTable.js for multiple column Pivot aggregations. I wish to show the negative numbers within brackets in red. Any ideas how can I implement this?

I tried

<style>
  .pvtNumber.negative { color: red; }
</style>

var config = {
  numberFormat: $.pivotUtilities.numberFormat("[#;[red](#)]"),
  // ...
}

No success thus far.