I didn't find any specific CSS designators for the area between the Members and the "user data", and I don't think there is any as you can see that the red extends past the green. Template edits could probably address that.
EDIT: Only "near" solution may be to use .p-nav-scroller
Code:
.p-nav-scroller {
background: yellow;
}
Would probably need some tweaking since the .p-nav-inner still extends past the green.
EDIT #2:
Change the margin-right:10px in the .p-nav-opposite segment to margin-right:0px; and you get this.