Scss: Adding missing z-index CSS variables.

scss/_dropdown.scss

@@ -18,6 +18,7 @@
 // The dropdown menu
 .dropdown-menu {
   // scss-docs-start dropdown-css-vars
+  --#{$prefix}dropdown-zindex: #{$zindex-dropdown};
   --#{$prefix}dropdown-min-width: #{$dropdown-min-width};
   --#{$prefix}dropdown-padding-x: #{$dropdown-padding-x};
   --#{$prefix}dropdown-padding-y: #{$dropdown-padding-y};
@@ -46,7 +47,7 @@
   // scss-docs-end dropdown-css-vars
 
   position: absolute;
-  z-index: $zindex-dropdown;
+  z-index: var(--#{$prefix}dropdown-zindex);
   display: none; // none by default, but block on "open" of the menu
   min-width: var(--#{$prefix}dropdown-min-width);
   padding: var(--#{$prefix}dropdown-padding-y) var(--#{$prefix}dropdown-padding-x);

scss/_offcanvas.scss

@@ -2,6 +2,7 @@
 
 %offcanvas-css-vars {
   // scss-docs-start offcanvas-css-vars
+  --#{$prefix}offcanvas-zindex: #{$zindex-offcanvas};
   --#{$prefix}offcanvas-width: #{$offcanvas-horizontal-width};
   --#{$prefix}offcanvas-height: #{$offcanvas-vertical-height};
   --#{$prefix}offcanvas-padding-x: #{$offcanvas-padding-x};
@@ -31,7 +32,7 @@
     @include media-breakpoint-down($next) {
       position: fixed;
       bottom: 0;
-      z-index: $zindex-offcanvas;
+      z-index: var(--#{$prefix}offcanvas-zindex);
       display: flex;
       flex-direction: column;
       max-width: 100%;

scss/_toasts.scss

@@ -1,5 +1,6 @@
 .toast {
   // scss-docs-start toast-css-vars
+  --#{$prefix}toast-zindex: #{$zindex-toast};
   --#{$prefix}toast-padding-x: #{$toast-padding-x};
   --#{$prefix}toast-padding-y: #{$toast-padding-y};
   --#{$prefix}toast-spacing: #{$toast-spacing};
@@ -38,7 +39,7 @@
 
 .toast-container {
   position: absolute;
-  z-index: $zindex-toast;
+  z-index: var(--#{$prefix}toast-zindex);
   width: max-content;
   max-width: 100%;
   pointer-events: none;