sudo nano /usr/share/themes/Human-Murrine/gtk-2.0/gtkrc

Near line 73, there is the following setting that has been commented out (disabled). I have no idea why the ubuntu devs did that, but it’s crappy usability. It currently looks like this:

#scrollbar_color = "#D6722D"

Make it look like this:
scrollbar_color = "#D6722D"

Also, near line 82, there is a setting for the scrollbar style: change it to whatever you want.

Now, I should be writing a bug report about this and how the ubuntu devs should change the defaults. Yeah, I should…