Theme: match message tray and panel box pointers

Panel box pointers were recently updated to match the mockups. Message
tray box pointers should look the same. Update the corner radius, stroke
colour and pointer dimensions to match.

https://bugzilla.gnome.org/show_bug.cgi?id=655627

data/theme/gnome-shell.css

@@ -1039,12 +1039,12 @@ StTooltip StLabel {
 }
 
 .summary-boxpointer {
-    -arrow-border-radius: 9px;
+    -arrow-border-radius: 8px;
     -arrow-background-color: rgba(0,0,0,0.9);
     -arrow-border-width: 2px;
-    -arrow-border-color: #5f5f5f;
-    -arrow-base: 30px;
-    -arrow-rise: 15px;
+    -arrow-border-color: #a5a5a5;
+    -arrow-base: 24px;
+    -arrow-rise: 11px;
 }
 
 .summary-boxpointer #notification {