Move offcanvas-backdrop css in offcanvas.scss

scss/_modal.scss

@@ -98,10 +98,6 @@
   &.show { opacity: $modal-backdrop-opacity; }
 }
 
-.offcanvas-backdrop {
-  @extend .modal-backdrop;
-}
-
 // Modal header
 // Top section of the modal w/ title and dismiss
 .modal-header {

scss/_offcanvas.scss

@@ -14,6 +14,10 @@
   @include transition(transform $offcanvas-transition-duration ease-in-out);
 }
 
+.offcanvas-backdrop {
+  @extend .modal-backdrop;
+}
+
 .offcanvas-header {
   display: flex;
   align-items: center;