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/_global.scss

@@ -1,6 +1,19 @@
+a {
+  @include dark-mode {
+    color: lighten($primary, 10%);
+      
+    &:hover {
+      color: lighten($primary, 20%);
+    }
+  }
+}
 
 body {
   font-family: $font-primary;
+  
+  @include dark-mode {
+    color: $body-color-dark;
+  }
 }
 
 // Prevents the page header from blocking view of
@@ -16,6 +29,10 @@ body {
 }
 h1:target, h2:target, h3:target, h4:target, h5:target, h6:target {
   background-color: yellow !important;
+
+  @include dark-mode {
+    color: black;
+  }
 }
 
 span.svg-icon {