fix modal zindex

assets/sass/_theme/_configuration.sass

@@ -149,6 +149,7 @@ $zindex-header: 52 !default
 $zindex-body-overlay: 51 !default
 $zindex-toc: 60 !default
 $zindex-toc-cta: 49 !default
+$zindex-modal: 72 !default
 $zindex-aside: 48 !default
 $zindex-footer: 70 !default
 

assets/sass/_theme/design-system/header.sass

@@ -69,10 +69,12 @@ body
             display: block
             opacity: 1
             pointer-events: inherit
-    html.has-offcanvas-opened &,
-    html.has-modal-opened &
+    html.has-offcanvas-opened &
         &::after
             z-index: $zindex-header + 1
+    html.has-modal-opened &
+        &::after
+            z-index: $zindex-modal - 1
     html.is-animating:not(.has-menu-opened) &
         &::after
             display: block

assets/sass/_theme/design-system/layout.sass

@@ -107,7 +107,7 @@ details
     top: 0
     width: 100%
     margin: 0
-    z-index: $zindex-toc
+    z-index: $zindex-modal
     &[aria-hidden="true"]
         display: none
     .modal-content