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

scss/_dark.scss

@@ -0,0 +1,17 @@
+$disable-dark-mode: false !default;
+
+@mixin dark-mode {
+  @if not $disable-dark-mode {
+    .dark-mode & {
+      @content
+    }
+  }
+}
+
+@mixin light-mode {
+  @if not $disable-dark-mode {
+    :root:not(.dark-mode) & {
+      @content
+    }
+  }
+}