How can I change it?

I've looked through the images directory within the relevant theme but have only found Progressbar.png and ProgressbarSmall.png which don't seem to be used by microb.

I've looked through the css file and the colours.config file but found no reference.

I'm looking through the gtkrc at the moment but a quick search of the file only came up with references to the two afore mentioned pngs.

Thanks
James