Scrollspy-style updating of hash and TOC highlighting

_sass/minimal-mistakes/_navigation.scss

@@ -507,6 +507,11 @@
     border-top-left-radius: $border-radius;
     border-top-right-radius: $border-radius;
   }
+
+  // Scrollspy marks toc items as .active when they are in focus
+  .active {
+    background: $active-color;
+  }
 }
 
 .toc__menu {

_sass/minimal-mistakes/_variables.scss

@@ -61,6 +61,7 @@ $muted-text-color: mix(#fff, $text-color, 35%) !default;
 $border-color: $lighter-gray !default;
 $form-background-color: $lighter-gray !default;
 $footer-background-color: $lighter-gray !default;
+$active-color: mix($light-gray, $lighter-gray, 50%) !default;
 
 $primary-color: #6f777d !default;
 $success-color: #3fa63f !default;