I have just done the following to the ambience theme:
$ sudo gedit /usr/share/themes/Ambience/gtk-2.0/gtkrc
edit line 11 & 12 to xthickness = 0 ythickness = 0
safe and close.
reload the theme by clicking on some other and than on ambience again.
See if this 'fixes' it for you, too.
I have just done the following to the ambience theme:
$ sudo gedit /usr/share/ themes/ Ambience/ gtk-2.0/ gtkrc
edit line 11 & 12 to
xthickness = 0
ythickness = 0
safe and close.
reload the theme by clicking on some other and than on ambience again.
See if this 'fixes' it for you, too.