classic: Improve menu separators

These were barely showing up in white-on-light-gray, so
turn them dark.

https://bugzilla.gnome.org/show_bug.cgi?id=697596

data/gnome-classic.css

@@ -3,7 +3,6 @@
 /* FIXME:
    - white edge highlight with text-shadow and icon-shadow for panel-button
    - better shading of the panel (dark 5%) - impossible without multipoint gradients, image-bg is a hack
-   - bubble menus: separators
    - startup notification spinner
 */
 
@@ -139,6 +138,15 @@
     color: #888;
   }
 
+  .popup-separator-menu-item {
+    -gradient-height: 1px;
+    -gradient-start: rgba(0,0,0,0.0);
+    -gradient-end: rgba(0,0,0,0.3);
+    -margin-horizontal: 24px;
+    height: 1px;
+    padding: 8px 0px;
+  }
+
   .popup-subtitle-menu-item, .popup-subtitle-menu-item:insensitive {
     color: #000;
   }