feat(theme): add dark mode

With version 3, Yuika now uses Shian as a basis of styling. Since
Shian was written with both light and dark themes in mind, this
means that the theme now is able to support switching between them.

This commit implements a dark theme toggle that will switch the
site between light and dark themes.

assets/js/dark-mode.js

@@ -0,0 +1,14 @@
+
+$(document).ready(function () {
+    $("#darkMode").click(function () {
+        if (window.document.body.getAttribute("data-theme") != "dark") {
+            window.document.body.setAttribute("data-theme", "dark");
+            $("#darkMode i").text("brightness_high");
+            $("#darkMode p").text("Light Mode");
+        } else {
+            window.document.body.setAttribute("data-theme", "light");
+            $("#darkMode i").text("brightness_low");
+            $("#darkMode p").text("Dark Mode");
+        }
+    })
+});

partials/navigation.hbs

@@ -6,4 +6,9 @@
         </a>
     </div>
     {{/foreach}}
+
+    <div class="menu__item" id="darkMode">
+        <i class="material-icons md-24">brightness_low</i>
+        <p class="menu__text">Dark Mode</p>
+    </div>
 </div>