changed zindex variables to use !default like everything else.

Author: holli

vendor/assets/stylesheets/bootstrap/_variables.scss

@@ -114,12 +114,12 @@ $dropdownDividerBottom: $white !default;
 // -------------------------
 // Used for a bird's eye view of components dependent on the z-axis
 // Try to avoid customizing these :)
-$zindexDropdown:          1000;
-$zindexPopover:           1010;
-$zindexTooltip:           1020;
-$zindexFixedNavbar:       1030;
-$zindexModalBackdrop:     1040;
-$zindexModal:             1050;
+$zindexDropdown:          1000 !default;
+$zindexPopover:           1010 !default;
+$zindexTooltip:           1020 !default;
+$zindexFixedNavbar:       1030 !default;
+$zindexModalBackdrop:     1040 !default;
+$zindexModal:             1050 !default;
 
 
 // Sprite icons path