🌓 Adds Dark Mode (#210)

* feat(dark mode): set up first set of colors

* feat(dark mode): define state colours

* 🧹

* feat(dark mode): variablify all teh wordz

* feat(dark mode): set colors on body

* feat(dark mode): replace hard coded color value

* feat(dark mode): 🌑

* feat(a11y): update link focus styles

- restores visible focus in windows high contrast mode
- increases visibility in boxes with bg colour

* feat(dark mode): enable postcss

* 💅

* feat(dark mode): add user control

* chore: use generic headline name

* feat(dark mode): hide switch until script loads

* feat(dark mode): increase link contrast

* add content warning to footer nav partial

* feat(dark mode): replace hard coded colour value in alertbox

* feat(dark mode): tone down text colour

* feat(dark mode): properly invert code elements

* 🧹 move box styles into own partial

* feat(dark mode): use darker colour as background for links

assets/css/structures/_definition-content.scss

@@ -81,7 +81,7 @@
     text-transform: uppercase;
 
     &--avoid {
-      --word-signal-color: $dark-red;
+      --word-signal-color: var(--clr-avoid-text);
 
       &:before {
         @include icon__avoid();
@@ -90,7 +90,7 @@
     }
 
     &--better {
-      --word-signal-color: $dark-green;
+      --word-signal-color: var(--clr-positive-text);
 
       &:before {
         @include icon__alt();
@@ -106,7 +106,7 @@
     }
 
     &--warning {
-      --word-signal-color: $dark-yellow;
+      --word-signal-color: var(--clr-warning-text);
 
       &:before {
         @include icon__warning();