Autoplot Forecast R

Suppose you have a time series model (ARIMA in this case).

fit <- Arima(WWWusage,order=c(3,1,0))
f <- forecast(fit, h=12, level=c(90,95,99))

And then you do this:

autoplot(f)

Now my question is, how can I change the color of the confidence intervals and get a graph like this:

Primarily I think is the colors, to make it change to be redder as it's closer to the start of forecasting and more yellow as time passes.

Thanks in advance!