Comment 1 for bug 422511

That is bug in the murrine engine and the trough-border was increased to elimate a bug in Firefox. I myself, personally, never use the murrine engine for theming my scrollbars, as you are very limited. The only detrimental effect is the loss of system-wide color theming to the scrollbars. Pixmap scrollbars scroll faster and smoother contrary to popular belief that the pixbuf-engine is slow.

The was referenced in an earlier bug report.

https://launchpad.net/bugs/327863

Also you check this thread:

http://ubuntuforums.org/showthread.php?t=1168984&highlight=humanity+theme