panel: fix part of the panel-corner-highlighting hack

The underline highlights on the panel menu items normally have a 100ms
transition between highlighted and unhighlighted, but the panel corner
graphics can't do that, so we hacked the Activities button and user
menu to have no transition. But in gdm mode, the user menu isn't the
rightmost item any more. Fix this by modifying the CSS from the code
instead.

https://bugzilla.gnome.org/show_bug.cgi?id=651299