Remove debug mixin, never used. Remove some obsolete colours.

assets/sass/_colors.scss

@@ -93,9 +93,4 @@
   // Highlight colours.
   --color-highlight:              var(--color-brand-alt);
   --color-mark-highlight:         var(--color-red);
-  --color-mark-bg:                var(--color-yellow);
-
-  // Misc colours.
-  --color-watermark:              var(--color-grey-200);
-  --color-debug-color:            var(--color-grey-200);
 }

assets/sass/_init.scss

@@ -12,7 +12,6 @@
 // Mixins and functions.
 @import 'mixins/breakpoint/respond-to';
 @import 'mixins/clearfix/clearfix';
-@import 'mixins/debug/debug';
 @import 'mixins/flex-calc/flex-calc';
 @import 'mixins/overlay/overlay';
 @import 'mixins/spacing/spacing';

assets/sass/_sass_variables.scss

@@ -72,6 +72,3 @@ $max-content-width: 1111px;
 // Setting this to a larger value than max-content-width allows
 // content with class "stretch" to stretch out to the page edge.
 $max-page-width: $max-content-width;
-
-// Show horizontal grid to help with debugging typography.
-$debug: false;

assets/sass/base/document/_document.scss

@@ -14,7 +14,4 @@ body {
   line-height: var(--base-line-height);
   // Accent colour for forms.
   accent-color: var(--color-accent);
-  // Output a horizontal grid to help with debugging typography. The
-  // $debug variable will toggle its output.
-  @include debug-grid();
 }

assets/sass/mixins/debug/_debug.scss

@@ -1,20 +0,0 @@
-// Output a horizontal grid to help with debugging typography.
-@mixin debug-grid() {
-  @if $debug == true {
-    $grid-height: #{$base-line-height + 'rem'};
-
-    position: relative;
-    // stylelint-disable-next-line  max-line-length
-    background-image: repeating-linear-gradient(180deg, var(--color-debug-color), var(--color-debug-color) 1px, transparent 1px, transparent $grid-height);
-
-    &::after {
-      content: '';
-      position: absolute;
-      bottom: -1px;
-      left: 0;
-      height: 1px;
-      width: 100%;
-      background-color: var(--color-debug-color);
-    }
-  }
-}