In case people are wondering how to use this feature, this is how I got it to work:
In the gtkrc file of the theme I was using, in the part after it says
engine "murrine" {
I added the setting:
progressbarstyle = 0 # 0 = plain, 1 = striped
This removed the candy-cane-likes stripes from the progress bar.
In case people are wondering
engine "murrine" {
I added the setting:
progressbarstyle = 0 # 0 = plain, 1 = striped
This removed the candy-cane-likes stripes from the progress bar.