🌓 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

package.json

@@ -60,6 +60,7 @@
     "markdown-it-prism": "^2.0.5"
   },
   "devDependencies": {
+    "autoprefixer": "^9.8.0",
     "ava": "^3.8.2",
     "concurrently": "^5.2.0",
     "eslint": "^6.8.0",
@@ -70,6 +71,7 @@
     "lint-staged": "^10.2.7",
     "markdownlint-cli": "^0.23.1",
     "parcel-bundler": "^1.12.4",
+    "postcss-preset-env": "^6.7.0",
     "prettier": "^1.19.1",
     "sass": "^1.26.7",
     "slugify": "^1.4.0",