Add global declaration to $prop assignment.

CONTRIBUTORS.rst

@@ -126,6 +126,7 @@ Contributors
 * Tomas Olander
 * Andrew Tork Baker
 * Vincent Audebert
+* Jack Paine
 
 Translators
 ===========

wagtail/wagtailadmin/static_src/wagtailadmin/scss/userbar.scss

@@ -298,11 +298,11 @@ $positions: (
             @for $i from 1 through $max-items {
 
                 @if $vertical == 'bottom' {
-                    $prop: 'nth-last-child';
+                    $prop: 'nth-last-child' !global;
                 }
 
                 @if $vertical == 'top' {
-                    $prop: 'nth-child';
+                    $prop: 'nth-child' !global;
                 }
 
                 &:#{unquote($prop)}(#{$i}) {