Using custom functions to access app theme

I am using a progress bar,

I want it to change the bar color depending on the result of the division of two numbers, to do so I created a Custom Function, and it works as it should, now I want it to use the Theme of my app, and I don't know how, this is what I have so far:

//... default flutterflow imports

Color getBarColor(
  double dividend,
  double divisor,
) {
  /// MODIFY CODE ONLY BELOW THIS LINE

  double res = dividend / divisor;

  if (res < 0.5) {
    return Colors.red;
  } else if (res < 1) {
    return Colors.yellow;
  } else {
    return Colors.green;
  }

  /// MODIFY CODE ONLY ABOVE THIS LINE
}

I tried (unfortunately) to access the theme using the line ThemeData theme = Theme.of(context);, but it didn't work, It seems custom functions are just static and pure functions that have access to no context.

So, my question is, how can I solve this inconvenience?

Thank you in advance y'all