Use proper variable for .dropdown-backdrop's z-index

scss/_variables.scss

@@ -359,6 +359,7 @@ $dropdown-header-color:          $gray-light !default;
 // Warning: Avoid customizing these values. They're used for a bird's eye view
 // of components dependent on the z-axis and are designed to all work together.
 
+$zindex-dropdown-backdrop:  990 !default;
 $zindex-navbar:            1000 !default;
 $zindex-dropdown:          1000 !default;
 $zindex-popover:           1060 !default;