add configuration for toc overlay color

assets/sass/_theme/_configuration.sass

@@ -246,6 +246,7 @@ $toc-title-font-family: $meta-font-family !default
 $toc-title-font-size: $meta-size !default
 $toc-title-font-size-desktop: $meta-size-desktop !default
 $toc-sticky-transition: 0.4s
+$toc-overlay-color: $body-overlay-color !default
 
 // Table
 $table-head-font-size: $h4-size !default

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

@@ -69,6 +69,7 @@ body
             pointer-events: inherit
     html.has-offcanvas-opened &
         &::after
+            background-color: $toc-overlay-color
             z-index: $zindex-header + 1
     html.is-animating:not(.has-menu-opened) &
         &::after