Dark Mode (#168)

* Implement toggle button

* Dark mode mixins

* Dark mode color design

* Disable dark mode for prints

* Initial dark mode design

* Fix light mode mixin

* Disable dark mode on print style

* Move dark mode color scheme below the default style to allow referencing

* Fix dropdown

* Fix device-based dark mode

* Dark mode index page design

* Responsive auxiliary nav with dark mode toggle

* Remove box shadow when on smaller displays

* Prevent default on toggle click

* Move attribution/licensing to README.md

* Fix header padding on index page

* Background color transition animation

* Remove margin on index when on mobile displays

* Remove neon border when on mobile

* Fix post-header margin on mobile

* Lighten card headers in dark mode

js/_dark.js

@@ -0,0 +1,25 @@
+(function () {
+  // Check if local storage is available and if dark mode is set; overrides everything else
+  if (window.localStorage && window.localStorage.getItem('dark-mode') !== null) {
+    if (window.localStorage.getItem('dark-mode') === 'true') {
+      document.documentElement.classList.add('dark-mode');
+    }
+  // Evaluate device preferences
+  } else if (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) {
+    document.documentElement.classList.add('dark-mode');
+  }
+}());
+
+$(function () {
+  $('.dark-toggle').on('click', function toggleDarkMode (e) {
+    e.preventDefault();
+
+    if (document.documentElement.classList.contains('dark-mode')) {
+      document.documentElement.classList.remove('dark-mode');
+      window.localStorage.setItem('dark-mode', 'false');
+    } else {
+      document.documentElement.classList.add('dark-mode');
+      window.localStorage.setItem('dark-mode', 'true');
+    }
+  });
+});