Add dark/light mode toggle to header

Author: sharadcodes

_includes/header.html

@@ -1,3 +1,19 @@
 <header>
   <a class="site-title" href="{{site.baseurl}}/">{{site.title}}</a>
+  <!-- dark/light mode -->
+  <span id="dark-mode-toggle" style="cursor: pointer">
+    <svg
+      stroke="currentColor"
+      fill="currentColor"
+      stroke-width="0"
+      viewBox="0 0 16 16"
+      height="1.2em"
+      width="1.2em"
+      xmlns="http://www.w3.org/2000/svg"
+    >
+      <path
+        d="M8 1a7 7 0 1 0 0 14A7 7 0 0 0 8 1zm0 13V2a6 6 0 1 1 0 12z"
+      ></path>
+    </svg>
+  </span>
 </header>