headsetcontrol-battery-widget/package/contents/ui/lib/helper.js

function batteryColor(batteryPercent) {
  const colors = [
    '#f00', '#e21c00', '#c83800', '#a50', '#8d7100',
    '#718d00', '#6fa100', '#5c8000', '#498000', '#080'
  ];
  let color;
  switch(batteryPercent) {
    case -1: color = "green"; break;
    case -2: color = "red"; break;
    default: color = colors[~~(batteryPercent / 10 + 0.5) - 1]; break;
  }
  return color;
}

function batteryLevel(batteryPercent) {
  if(batteryPercent == -1)
    return "charging";
  else if(batteryPercent == -2)
    return "not connected";
  else
    return batteryPercent + "%";
}