How to assign a unique color to widget-box title borders of a specific sidebar?

All of my widget-box titles currently have a border-bottom (underline) color of gray, in every sidebar, but I would like to change the border-bottom color of the inner-right sidebar widget-box titles to a different color; however, there doesn't seem to be an option for that in the Atahualpa theme panel--only for assigning unique colors to sidebar borders and links. How do I assign unique colors to widget-box title borders based on the sidebars (in this case, the inner-right sidebar)? Basically, I would like for all of the border-bottoms of the widget-box titles in the inner-right sidebar to be a color of my choosing. An example would be this website's sidebar-title borders; theirs are one color on the right but a different color on the left: http://www.guardian.co.uk/

Thanks,
Rachel