[gtkmm] Colouring progress bars in gtkmm 1.2?

[Matthew Loh <lohm mail lafayette edu>]

First off, thanks for taking the time.

My question is how one goes about changing the colours of progress bars
based on the amount of progress that has been accomplished - e.g. red when
the bar is at 0 - 30%, yellow at 30 - 60%, green at 60 - 100%

I was thinking maybe something to do with styles, but the gtkmm 1.2
documentation is cryptic as ever.

Thanks,

Matt Loh