🌓 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

11ty/shortcodes/renderDefinitionContentNextEntries.js

@@ -19,7 +19,7 @@ module.exports = function definitionContentNextEntries(
 
   return `<section class="definition-navigation definition__further-definitions_nav">
     <h2 class="visually-hidden">Browse</h2>
-    <div><h3 class="definition-navigation__sub-headline" id="context-nav-previous">Previous words</h3>
+    <div><h3 class="smcp-headline" id="context-nav-previous">Previous words</h3>
       <nav class="definition-navigation__nav" aria-labelledby="context-nav-previous">
         ${
           previous.length
@@ -29,7 +29,7 @@ module.exports = function definitionContentNextEntries(
             : ''
         }</nav>
     </div>
-    <div><h3 class="definition-navigation__sub-headline" id="context-nav-next">Next words</h3>
+    <div><h3 class="smcp-headline" id="context-nav-next">Next words</h3>
       <nav class="definition-navigation__nav" aria-labelledby="context-nav-next">${
         next.length
           ? `<ul class="definition-navigation__list">${next