make it a var

Author: mdo

scss/_variables.scss

@@ -608,6 +608,8 @@ $nav-pills-border-radius:           $border-radius !default;
 $nav-pills-link-active-color:       $component-active-color !default;
 $nav-pills-link-active-bg:          $component-active-bg !default;
 
+$nav-divider-margin-y:              ($spacer / 2) !default;
+
 // Navbar
 
 $navbar-padding-y:                  ($spacer / 2) !default;

scss/mixins/_nav-divider.scss

@@ -2,7 +2,7 @@
 //
 // Dividers (basically an hr) within dropdowns and nav lists
 
-@mixin nav-divider($color: #e5e5e5, $margin-y: ($spacer / 2)) {
+@mixin nav-divider($color: #e5e5e5, $margin-y: $nav-divider-margin-y) {
   height: 0;
   margin: $margin-y 0;
   overflow: hidden;